If your new to growing stop before you use any fertilizers,forget 90% of the shit you read on this site,the biggest problem new growers have is doing too much & thinking more is better when in fact less is better,keep it as simple as possible & you'll have an easy time growing,start adding shit like fertilizers & now things start to get complicated & you run the risk of fucking the entire grow up from too much love,everybody wants to do the most they can to increase harvest weights,its human nature but not the right thing for newb's to do.
Case in point,your only 3 weeks into the grow & your allready wanting to add fertilizer,unless the soil your using is complete garbage & depleted of all nutrients it should easily carry your plants into the 2 month mark without adding a single thing,wait until you see a slight yellowing of the leaves & that will tell you it's time to add Nitrogen,when looking at fertilizers the NPK rating will tell you how much of what nutrient is in the mixture,for nitrogen content you want a fertilizer with a high N in the NPK rating,use a fert at that point with a rating of 30-10-10 at 1/4 strenght once a week up until budding cycle,no need for ultra expensive hydro nute's or majic sauces that promise all kinds of increased shit,most of those promises are pure bullshit,miracle grow/peters brand/jacks brand ect are very good fertilizers but they are very strong & need to be used at 1/4 strength.
It's too early to use any fertilizer on your plants,do as little as possible your 1st grow,then in your 2nd grow take what you've learned from a completed grow & take it to the next level with each successfull grow,dont rush shit.