The problem is, you're trying to solve this problem as a box i.e. you want to do all your improvement in this box. But you have 3-way switches. You need to solve each 3-way switch group as a group.
Indeed, the right 3-way is totally separate from the others, and should not be cross-connected. If your box allowed separators you could even put a separator in there.
Rightmost 3-way
On the rightmost 3-way, this is location is the "remote" and both power and lamp cables come into the other end. That is where the smart-switch should go. Lutron Caseta all operates wireless, and their remote switches don't even use wires, so if you put the remote here you simply won't use the 3-way cable at all.
If you want to use a mechanical switch, then you'll need to follow their guidance for wiring one, which is fairly illogical actually. What it's trying to say is you connect a remote switch between the Caseta's black wire and its blue wire. If the switch is thrown, Caseta treats that as an on/off switch command.
So in this case, the Caseta at the far end would send always-hot on the black wire to this location. That goes to the black screw on the 3-way. From a brass screw goes the red wire, back to the Caseta's blue wire. White is unused. The other brass screw is unused.
The rest of it
The left half of the box is complicated. The top cable goes to a lamp, its wires are switched-hot (black) and neutral (white).
The top left cable has 2 3-way travelers (white and red) and always-hot (black) heading off to a remote 3-way switch.
The bottom left cable is kind of interesting. It goes to the lamp controlled by that 3-way (that's the red wire), and it is also the source of power for the box (black=always-hot, white=neutral).
Anyway, here, you would install the Lutron Caseta master, adding its always-hot to all the wires in the black bundle, and adding its neutral to the neutral bundle. The "Load" would go to the red wire from the lower left cable.
As it happens, the remote 3-way is already set up in an advantageous way, with red and white going to travelers and black going to common and that's already wired to always-hot. So leave the black where it is. Tap red for the Caseta's blue wire. The white traveler is capped off.
This is not as hard as it looks
We can tell from your photo that the far left cable provides the always-hot and neutral for the lighting branches on the left and center switches, which leave via the two middle cables, while the far right cable is for the 3-way circuit and thus needs to be left alone. Furthermore, we can see the neutral bundle for the two outdoor light circuits in the back of the box, running left to right.
This means that putting both outdoor lights on the same smart-switch is easy. The neutral bundle gets pulled out from the back of the box and the switch's neutral gets added to it, with a pigtail of appropriate gauge wire used to make the connection. Likewise, the existing ground bundle is pigtailed to the ground screw on the switch, and another pigtail is run from that ground bundle to the ground screw on the 3-way switch to the right, as the grounding in this box appears to be improper.
Then, we can cut the wire that connected to both the left and center switches (on their lower screws in your photo) back to just before the first stripped section, strip an end, and connect it to the LINE terminal on the switch. The two wires going off to the outdoor lights are then wirenutted to a pigtail that connects to the LOAD terminal on the switch. Get a suitable faceplate (3-gang with a blank in one position, a decorator or toggle in the second, and a toggle opening in the third -- modular faceplates are also an option for this, or you could use a switch-filler in the existing faceplate if your new smart switch uses a toggle style handle), button it all back up, turn the breaker on, and enjoy your new smart switch!
Best Answer
Current flows in loops
You know, with an air tool, you only need one hose. When the tool is done with the air, it can just release it into the air! Hydraulics could work that way if we were fish. But since we're not, hydraulic tools need a return line.
Electrical needs a return line too, and it's even more important because electrical power works on the difference of forces. So the return line has the same force as the supply line, actually (that's why we insulate it and keep it separate from safety ground).
So a load connects between hot and neutral (in our nomenclature.)
Now a dumb switch is human-powered. It needs no power of its own. So a dumb switch simply connects (or does not connect) an always-hot wire to a switched-hot wire. Those are the 2 wires in the switch loop before you.
So what options do you have?
Replace the "switch loop" with a /3 cable
The cable run between lamp and switch does not have neutral. We change that if we have ready access.
Unlikely, but if it's in conduit with loose individual wires, a white THHN neutral wire can simply be added, and done.
Generally, pre-2011 wiring will be a /2 cable meaning 2 conductors (black, white) + safety ground. Change that to a /3 cable which has black, white, red. You cannot simply toss in a loose single wire, because all related conductors must be in the same cable. The old cable can be up-cycled to other tasks if it's removed carefully.
In the /3 wiring, you must use white for neutral (it's not that way now). Then I advise using black for always-hot, and red for switched-hot (to the lamp). If you do that, the colors will probably match up with your smart switch.
Select a no-neutral smart switch (leaks through the lamp)
This is the option if you don't have a functional grounding system. This works the way traditional dimmers work - it relies on the very low resistance of incandescent bulbs to "leak" a small amount of power through the bulb to power itself.
This won't play well with CFLs or most LEDs, however that can be solved by adding one incandescent bulb, or paralleling a Lutron LUT-MLC "incandescent simulator" into the lamp wiring.
Select a no-neutral smart switch (that bootlegs ground)
You should never do this on your own. This, plus a trivial wire problem, would render the grounding system itself lethal. Not what you want!
And this only works if your grounding system is in good order clear back to the panel.
UL (Underwriters's Laboratories) writes the rules that appliances must follow to be certified as safe. Under very narrow rules, they allowed smart switches to power themselves by bootlegging the ground wire as a substitute for neutral. The rule limits current to 0.5 milliamps (not harmful if the above happens) and too low to trip GFCIs.
However NFPA (who writes the National Electrical Code) does not like this method, and outlawed it as of NEC 2020 (or to be more precise, outlawed sale of the devices as of Jan 1 2020). The good news is, only one US state has adopted NEC 2020. So they should still be available, but don't count on that supply to last.