Are you using a dimmer?

Are the bulbs enclosed in the fixture? And if so, are the bulbs rated to be used in enclosed fixtures?

No dimmer. No bulbs, just 20 or so tiny leds on a circuit that come in an enclosed fixture.

I would suggest that it's a problem with the fixture itself. I would try to return/replace it.

Odd that it's just the left unit that keeps failing.

Here's an irresponsibly wild guess. The light manufacturer switched to a different supplier for the LED modules partway through production, and you were unlucky enough to get one which had 2 old and one new-design module. The replacements have all been the new design, so they let their smoke out too.

As I say, wild guess, but I don't have another answer.

Maybe it's not too late for you to take the entire fixture back to the store and get something different.

Yep, swap it out. LEDs die primarily because of heat - manufacturers generally figure this out and upgrade to larger heatsinks with the next iteration of their products. If you open up the working ones you may find them different inside.

That's the other reason I was thinking of -- somehow the leftmost unit is getting less cooling than the other two.

The dirty secret of most LED lighting is that the cheap ones especially are not as efficient as the LED reputation would suggest.

For example, I see the orange store's website offering a 3-light Feit LED bathroom fixture that eats 26 watts to produce 900 lumens, a pitiful 35 lumens per watt.

I have a few Halo disc LED lights. I like the light they produce, but not the efficacy: 54 lumens per watt.

OTOH your typical $1.50 retrofit compact fluorescent may have its faults (don't we all), but will give 60-70 lumens per watt. Good T8 tubular fluorescents with electronic ballasts can easily manage 80-85 lumens per watt.
Any watts that go in as electricity, but don't come out as lumens, get turned into waste heat. Inefficient little LEDs need big heatsinks to dissipate the heat. If they don't have them, or the heatsinks don't have enough air flowing over them, then the LEDs will fail in a hurry.