Two light fixtures in bathroom with 3 bulbs each. They can can 100 watt incandescent, I have 800 lumen LED, and there is a dimmer switch. In both fixtures, going left to right, the first two bulbs light and the 3rd does not.
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 & require very tight limits to operate. Meaning, the first 2 bulbs are reducing the voltage & 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 & see if it lights, it should. Then, try incandescent & LED combo's to see what does & doesn't happen.
You may be able to adjust the dimmer, very few have this ability & 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.