+1 no effect. also, no need to transfer to a secondary just to dry hop, just toss em in the primary after you've given the yeast a chance to settle a bit.
It's in a plastic primary 3 weeks total in plastic wont hurt it ? And the new hops will just sit on the old one won't they should I mix them in gently I wouldn't want them to not soak
3 weeks in plastic won't make any effect. you don't need to worry about that having a negative effect for months. whatever hops were in there previously should have settled out by now, no need to mix.
you can certainly still rack to secondary if you want for it, IMO its an unnecessary effort and risk of contamination/oxidation