If they were planted recently, it's just a case of the other two not establishing properly yet. You are supposed to leave rhubarb plants for a year anyway before pulling stems to get the roots well established.
If they are just weak plants though, and don't pick up with watering and feeding, I'd propagate from the vigorous plant that is doing so well, or buy another plant.
Check that they aren't simply sitting in water as although rhubarb likes lots of moisture, the plants won't like being waterlogged before they've established a root system. If they're still quite small, you could dig them up, check the state of the roots, and put in large pots for growing on until they are a bit stronger and you can plant them in the garden again.