My 3 years old threw up few times today.she had 100.3 fever before her nap but not before or after that.is it a virus or something else?

Can't tell: It's impossible to tell without a physical or work up. One thing i can say is if it resolves in a couple of days don't worry about it. If it gets worse or does not resolve see a doctor.

Viral illness: This sounds like it is most likely a viral illness. Try giving your child small amounts of gatorade frequently. If that is tolerated well, you can slowly advance the diet to a bland diet, i.e. Banana, rice, applesauce, and toast/crackers. If needed you can give Acetaminophen for the fever. If illness does not resolve, you should see your personal doctor or have your child seen in the er.