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.