when to feed what to use?

I'm not familiary with your soil OR your nutrients but at Week 3, you could be giving her 1/4 strength grow nutes and she would respond nicely I think. I usually start 1/4-strength grow nutes about week 1 or 2 depending on how strong the seedling is... Autos will go into flower around week 4 or 5 and you can start bloom nutes (and calmag if necessary) when they're in pre-flower. But at the moment, these ladies surely need some grow nutes - just introduce them slowly - like 1/4 strength first feed, 1/2 strength second feed, then full strength... Good luck!