My 3 yr old have high fever, runny nose, vomiting, small rash on trunk, and is irritable.. sister have fever but no vomiting.. wht is this? wht to do

Sounds like viral: Although, it does sound like a viral infection, 3 days of fever is the most concerning factor. Vomiting in the 3 year old is not good either as it may cause dehydration along with fever. I'd take him to the nearest er for a check up.

Agree: It is the time of year that many viruses are circulating. Nothing replaces a face to face visit to assess what is going on. Whenever there is a rash involved, it is best to be seen as rashes cannot be adequately diagnosed without being seen.