I grow in 3 or 5 gal bags with FFOF after starting germinated seeds in Happy Frog. I also use the 3-part nutes you’re ordering. I recommend that you get a bottle of cal-mag. Mine is from General Hydroponics. Cannabis is calcium and magnesium hungry. Just use it per bottle instructions.
No rush on the nutrients. The FFOF should carry you for a couple more weeks, it’s pre-amended a little. Water-only for another week or two, then start the Grow Big (which is nitrogen heavy).
Since it’s a 1st grow, I suggest you stick with the “soil” feed chart from FF. Start grow big at about 2/3 strength and adjust from there.
If you can find a small amt of perlite to mix into your 3-gal pot it’ll help improve your soil drainage and post-fluids dryback. But watch for the level of coarseness. You want it smallish like this,
View attachment 2187384but not fine. I add about 5-10% but others here say they go heavier. I bought a lifetime supply on Amazon (4 cubic feet). Way too much, but it was a great price and just the right coarseness.
Do you have a drip tray and pot elevator (a riser) to let the pot drain? You’ll also want a set of measuring spoons for your nutrients—my wife prefers I use my own set for measuring out guano and worm dookie nutes.
EDIT: I prefer to measure my pH liquids and cal-mag using a 5 ML dropper, rather than measuring spoons.
Good luck friend!