3 yr old had fever 100.5 armpit for 3days and 102.5 day 4&5 then it went away he's still congested could it've been strep. On day 1 rapid test was neg?

Probably Not. If a throat culture was done and negative, highly unlikely. See my other answer to your fever question. If a throat culture was not done at this age, strep is not totally ruled out.
Probably not... Though the rapid strep test can sometimes miss things, it is pretty good. Most likely a culture was also obtained at the same time which is more sensitive. If that comes back positive, your health care provider will contact you to start antibiotics. In general, you don't get a lot of congestion with strep throat - this sounds more like a virus.