I would look for a voltage drop caused by a loose connection in a junction box they tap from or service panel if they all do it, or individually if only certain ones do it. Putting a 100watt lamp in a 40watt socket is a safety(heat)issue (how much heat the fixture is rated to handle), if the fixture cant throw off the heat it would affect the lamps life some, but not that fast.
I am not certain, but the likelihood of finding a dimmer in a single gang with two separate switches is unlikely. In addition to the electronic components needed by a dimmer, it also needs a significant heat sink.
You could replace the existing single gang box with a double gang box, replace the triple switch with a double, and run the LED wires to a spearate single gang dimmer (LED rated).
Before working on the box, turn off the power to the box at the breaker panel. Usually, all switches in a single box are on the same circuit, BUT TAKE NO CHANCES. Make sure each wire is not live with a non-contact tester.
Depending how your existing box is mounted, you may be able to simply cut sideways into the plaster/wallboard sufficently to fit the double gang. If your existing box is nailed into a stud, you should be able to access the nails and remove the old box. If the old box uses a an extended strap to hold it in place. you might need to take out some wallboard above and below the box.
The new box can be screwed into the stud on one side and supported on the opposite side with either built in support brackets or with sheet metal support straps that lock it to the plaster/wallboard. If the box is free-standing (not on a stud), it can be replaced by an old-work bouble gang box that has built in support brackets for both sides.
As you are working on the existing box, be careful of the existing wires.
Best Answer
No, this really won't work. Suppose you have 2 dimmers at 20% and 50%. You are hoping the effect will be multiplicative, that is, 50% of 20% is 10%. It won't.
Two SCR dimmers in series will, at best, give you the lower dimming level of the two, i.e. 20%.
A PWM dimmer, commonly used on dedicated LED circuits, pulses at its own high frequency. If you have two of them, they will pulse at different frequencies. This will create a much lower "beat frequency", in music this is what makes it annoying when instruments are out of tune with each other. In light, this would look like pulsing or shimmering. You won't like it.
Actual LED emitters can be dimmed very effectively to absolutely any light level. Turn the LED on for 10 microseconds and off for 990 microseconds, you will get 1% brightness - they can be controlled that precisely. It's a lot harder in built commercial products, which have to work with a variety of existing dimmer technologies made for incandescents, and also hit a "price point" that will make you snatch it off the shelf at Home Depot. The result of these compromises is lousy dimming range.