How Big Of An RV Do I Need For 3 People?

It depends on the lay out of the RV.  Some have bunk beds that will sleep 4 in twin bed accommodations.  Some will sleep 2 in a queen size bed and 1 or 2 on the couch.  Another option is 2 in the bedroom, one on the couch  and one on a fold down twin size bed that is located over the couch.  For 3 people on short weekend or a one weeks trip,  I would not consider anything less than a 19 foot RV.  If you intend anything longer than 2 weeks, (minimally)  21 foot would be my choice.  The biggest you can afford is the way to go.  No matter what they tell you and no matter how nice the RV and no matter what niceties it has...it can get crowded in a hurry.