If it is only three fish that you lost I wouldn't panic just yet. I know we had big swing in water temps here in Nebraska in the last 10 to 11 days that stressed my fish. My evening surface temps went from 68 / 70 degrees into the upper 80s in just a few short days. My GSH were about ready to start spawning but now water temps are probably too warm. I had 9 SMB floaters six/seven days into this temperature jump that was probably caused by a combination of things but appears to have settled down. Work on getting aeration going and keep your fingers crossed.