Like you I started with three rats - two females and a male. Every one of them sneezes although they have not shown any other sign of illness, and from what I have read this is fairly common behavior from "pet store" stock. It can be caused by low grade infection, dusty bedding, strong smells, etc. Key is to keep a close eye on your rattie and if they get worse, see a vet!

The male and one of the females were both very aggressive when I first got them. I invested in a pair of cut and puncture-resistant gloves (about $17 on Amazon) for handling sessions and made a point to give attention during bin cleanings. Now neither will mouth, nip, or bite. She lets me take babies right out of the bin without making a single aggressive move.