Sounds like the ones that are left are pretty healthy. The thing that sends up a red flag is the heat lamp. Why are you still offering heat? 3 1/2 month olds do not need any heat, unless it is getting below zero. Adding heat at this age only make them sickly.
Doesn't sound like they have any intestinal issues, however they could have some other bacteria anywhere in their bodies. There is nothing wrong with keeping them on a solid floor as long as you keep it very clean. You have 16 square feet in this box and 6 birds. This is the bare minimum space you want for that amount of birds. So I am guessing your issues are stemming from keeping too many birds in a very small space, (Gambels need at least 4 square feet per bird) causing disease and mental stress, and you are not allowing them to develop properly with added heat.
I suggest you start them on some antibiotics. Duramycin is a good all around broad spectrum drug and will cover most gram negative and gram positive bacterias. It is mixed 1 teaspoon per gallon or water. It must be mixed fresh daily. Give this to them for 7 full days. After this duration, I suggest you get some probiotics and add this to the water for at least 2 weeks after. This will not only replace the good bacteria that was lost during treatment, but will boost the immune system for over all good health. Offer up probiotics once a week there after.
At this age they are also molting into their adult feathers. This can cause stress to the birds and coupled with the things above, they are probably a bit internally stressed. The birds are only as strong as their weakest link. They should be fine once you get them healthied back up.
Good luck!
