Several possible causes but they all boil down to 'stress'. Stressed plants aren't inclined to produce as that only adds to their stress. So once you ID the cause of the stress and eliminate it the problem resolves itself.

In this case it could be over-crowding (3 plants in 1 container would normally be considered over-crowded no matter the size of the container), lack of sufficient nutrients, insufficient sun exposure (shading), rootbound plants, etc.

Dave

Go buy "blossom set" spray and spray it on the other two plants. That will fix it.

Sorry to disagree but blossom set sprays only work during cool weather and usually result in deformed tomatoes. They don't fix the problem that is causing the blossom drop. See the FAQ here on Blossom Drop and its causes. Blossom Drop has many possible causes as mentioned earlier.

http://faq.gardenweb.com/discussions/2765999/why-are-the-blooms-on-my-tomato-plant-dying-and-falling-off

Dave

The OP has Tomatillos, not tomatoes. Is culture the same as for Tomatoes?

The fact that the two which are not fruiting are taller, more robust and darker green makes me suspect excess nitrogen. Odd if they are all in the same container but has any fertiliser been applied and if so was it given in an unequal way?

Another possibility is that the tall pair are simply genetically slightly different and will fruit in due course. I've only ever grown them once since they failed to ripen any fruit in my climate. There was a good deal of variability in the seed grown plants.

They have different culture needs than tomatoes and different reasons for blossom drop. If one is fruiting, they should all be fruiting. If two aren't, it's likely a pollination issue and blossom set spray will fix it. They can't self-pollinate and wind doesn't do the trick. The bees or whatever probably are stopping at the big plant first then the little plant--that would result in uneven fruit production.

Marianne - it's the runtiest plant which is fruiting so the bee theory doesn't hold up.

No, that's how they look when they fruit. They get scraggly and sad looking. That's normal.

Tomatillos are just weeds that we happen to tolerate. They have totally different growth patterns than tomatoes, like they need pollinators. If one is fruiting and the others aren't, it means conditions in general are right, just the other two haven't had the right kind of pollination/hormone trigger (I swear, this is a thing with them). Blossom set spray will work. Spray it and then leave the weeds alone and you'll come back later to find them covered with fruit.

Marianne, I wasn't judging by the pictures. I was reading what the OP told us. 'The two that aren't fruiting (on the right) are much larger and have thicker stems and darker leaves. The fruiting plant is smaller and more spindly.'

So your theory that 'The bees or whatever probably are stopping at the big plant first then the little plant' doesn't hold water.

I have grown tomatillos and tomatoes and am aware they are different. My question was rhetorical since both you and digdirt appeared to have moved on to discussing tomatoes rather than tomatillos.

Sorry didn't mean to imply I was only discussing tomatoes. The issues I discussed above can apply any blooming/fruiting plant, not just tomatoes and not just tomatillos.

I too grow both and while there are some cultural differences it isn't a drastic amount and many aspects of the 2 plants are the same - including the causes of blossom drop. Personally my tomatillo plants don't get scraggly or sad looking and they sure don't look like weeds and all my experiments with the various blossom drop sprays have supported what i said above. But then my climate and the effect it has on them is quite different from that in zone 10 (have no idea where the OP is located). They look different from tomato plants in ways but look and act just like them in others.

As to the self-pollination issue, it is hotly and often debated and research from Purdue University has the best info on it. https://hort.purdue.edu/newcrop/morton/mexican_husk_tomato.html

Studies show that some varieties are readily self-pollinating while others are not. Some varieties require a wider range of cooler temps to self-pollinate, and some plants it seems will never produce fruit regardless of the growing conditions. So there are no absolutes about them.

However 3 plants in one container (without knowing if it is an exceptionally large (ie.25 gallon) container is normally over-crowding. And as floral pointed out 1 large bushy plant with another small plant is often indicative of excess or uneven distribution of N in the container. That 2 holds true for any blooming/fruiting plant.

Dave