I will take a shot at what is wrong.

If you used a recommended dosage of nutes when you first put those plants in the 6 gallans 3 weeks ago, I highly doubt it is a nutrient deficiency. I would guess it is an oxygen issue with the roots. I would guess that the water is over 70 degrees and you are using a smaller air pump. Is the one plant that is having the issue in the middle of the tub and the other two on the outside with the air stones directly under them? I would add air, probably double it.

You might as well just do a nutes change though if you think it might be a nute deficiency. That doesn't take much work and should be easy to find out if that is what it is.