Problem with flock of chicks

For the past week I've had numerous problems with my flock of chicks. The chicks are about 3 months old and I have aproxametly 15 chicks. They are a mix of buff brahma bantams, Mille fleur bantams, black austrolorp bantams and 1 partridge rock bantam, the problems started out with 1 of the brahmas having foam starting to building up in 1 of the eyes. And if you picked him up, liquid would start coming out of his mouth. He eventually got better but a few days later he died. My black austrolorp hen, ( that was hatched under a broody hen ) had no problems when outside in the chicken tractor. but when I moved her inside the coop for the cool tempetures. She seemed to have splayed leg. Currently she has no problems and is running around, eating, drinking. My last problem was with my partridge rock bantam hen. I'm not really sure what happened to her or what caused it. But she sure deteriated fast. It started with her not moving around very much, and her tail drooping down to the ground. And then just yesterday she seemed to get splayed leg or she was just laying down. But when I went to put the chicks in last night I found her dead. I have a heatlamp in there. To keep the water unfrozen. But it is still available for them to go under. I'm wondering if the heatlamp is causing all the problems because it is keeping their bodies from climatising to the cold weater here in northern canada. 1 other thing that I can think of that might be causing problems is that I dusted them all about 2 weeks ago. Because they had lice and I don't want any lice in the coop during the winter. And the dusting powder might be causing problems to their health. So if any body knows what might be the name of these problems. Or what I'm doing wrong.please tell me because I don't want this to spread to the rest of my flock.