I'm sure that you realize by now, that the one stalk was dieing. It's a challenge to maintain plants (that would ordinarily prefer to be planted in a solid medium) in a healthy and robust fashion in water. To have one of a group give up the fight would seem kind of normal to me.

For the others, remember that the plants need to be able to absorb oxygen at the roots. Keep the water changed, and clip the roots every once in a while to generate a new flush of roots.

The thing is, all three of them are planted in the same pot! It is all dead now, except for the last tip of the stalk that I salvaged and stuck in another pot.