That sounds like a short to me, and a potential fire or electrocution hazard. Leave the breaker off while you fix this. With a 3-way light switch, you'll have two possible paths between the switches (which is why there are 3 screws + ground, 1 line/load, and 2 switched). My guess is that one of the two switched wires is grounding itself. Remove the switch that you worked on, and see if the ground wires are coming into contact with any other wires.
Smart switches are not equal partners like old style 3-way switches. They are "master" and "remote".
This model of smart switch is a "master" and requires certain models of "remote" which require "neutral" (white wire) to be present in both switch boxes. That is not always so, as the electrical code only recently started requiring it.
So I'd first check the other box - if there's no neutral in it, you cannot wire a remote (though wireless options may exist). Useless old work can be dangerous, so tidy up by removing the 3-way switch, wire-nut the common to one of the messengers (trial and error will reveal which), cap the other one off, and put a blank wall plate over the hole.
If a white wire is already wired to a terminal on the old 3-way switch, bad news - that is NOT a neutral, but one of the messengers (the two inter-switch wires). Before the code change, they often used common 14/2 Romex for messenger, and one of the wires is white. They're supposed to paint or tape it.
If there is a neutral in both boxes, you can wire a remote. First, figure out which box is upstream (closest to the service panel aka breaker box). It will contain a wire that is always hot - regardless of the position of any switch.
Then review the diagram (which you already linked) showing you the wiring scheme for whichever remote you bought. You will need to re-use both of the old messenger wires.
https://drive.google.com/file/d/1pdPZeA9_Oi-rYKF0LQrkVhmVpo4DmJJ2/view?usp=sharing
Best Answer
I'm not an electrician, and I'm not familiar with the National Electrical Code, so this answer may contain inaccuracies.
Everything seems to make sense, but I see some potential problems.
In this diagram, all of the black wire nuts are always-hots. So are all of the black wires, except for the one coming from hall light #1. The white wire nuts in the upper boxes, and all of the wires connected to them, are neutrals. The white wire nut in the switch box, and all of the wires connected to it, are switched-hots. Each of the hall lights is connected to one switched-hot and one neutral, just like you want. All of the other cables exiting the diagram carry one always-hot and one neutral.
Here are the potential problems that I've noticed:
First, on hall light #1, the black wire is connected to neutral and the white wire is connected to switched-hot. It's probably supposed to be the other way around. If the light fixture has an Edison socket and it's wired the wrong way around, then the hot wire will be connected to the easy-to-touch part of the socket instead of the hard-to-touch part, which increases the risk of electrocution.
Second, the hot wires form a loop: the black wire in the upper left box connects to the switch box, where it connects to the upper right box, where it connects back to the upper left box. I believe that's a code violation, but I'm not able to find a source for that.
Third, the hot path and the neutral path for hall light #2 are different: the hot wire goes through the switch box, whereas the neutral wire goes through the cable that directly connects the two upper boxes. This will cause those wires to produce a stronger magnetic field than usual whenever those lights are turned on. If you have any audio equipment in your home, it may pick up that magnetic field and make an unpleasant noise (mains hum). If you don't have any audio equipment and you don't plan to get any, then mains hum won't be a problem for you. However, A. I. Breveleri pointed out in a comment on this answer that these magnetic fields can also cause inductive heating in nearby metal objects.
(When a wire carries a current, that wire will always produce a magnetic field. However, if you have two wires right next to each other carrying the same amount of current in opposite directions, then their magnetic fields will mostly cancel out. That's one of the reasons why hot and neutral wires are usually run right next to each other.)