Did you see pediatrician? what did pediatrician says?
If you have not bring him to see pediatrician, better bring him soon....meanwhile keep him hydrated as vomit & diarrhea will cause a lose of water....i am not sure if 3mths can give him gripe water or not or glucoline to at least keep him hydrated...but try if you can to spoon feed him a bit of water.
Btw, you say he is on soy milk, how about trying other brands of FM? NAN is the closest to breast milk, mayb you want to try a small can?