Only 2 of the 3 LED bulbs lite up
If all 3 incandescents worked previously without issue (so you thought), then I'd have to say that the dimmer is limiting the current. LED's are very sensitive &amp; require very tight limits to operate. Meaning, the first 2 bulbs are reducing the voltage &amp; amperage to leave the third bulb unable to activate.

Your 3rd incandescent may have been dimmer than the other 2 or all 3 may have stepped down equally. Which leads to, put an incandescent in the 3rd sockets &amp; see if it lights, it should. Then, try incandescent &amp; LED combo's to see what does &amp; doesn't happen.

You may be able to adjust the dimmer, very few have this ability &amp; you'll need the dimmer's manual to do it. But, I'd replace the dimmer with a regular switch to confirm that the dimmer is the problem. If so, then get a new dimmer or keep getting them until one works. I say that because I doubt there's anything on the box to indicate any difference from 1 dimmer to another.