Dont think there is a problem with washing a car that has fresh paint on mate, just use proper car wash and a wash mitt (NOT A SPONGE!).
Its polishing you cant do until the paint is properly set. I washed my newly sprayed bumper a few days after getting it done, they should harden it for you before you pick it up, you just have to leave it to properly set before a polish, and I think even then 3 weeks is more than enough