You're doing fine. I typically start my seedlings off in 3 gallon pots and I've never seen any evidence of my plants being stunted because of it.
Calmag right now is important, as well through veg and mid flower. You could have gotten away with just
calmag and a root stimulant until this point...adding nutes from the start is just a preference. Another thing, your plants can get away with just a foliar right now...
calmag has enough nitrogen for this stage. You're doing well, keep the humidity up, and go find something else to do with your time lol. You're at a space where you can ph a nice batch of nutes, fill your pots until you have some runoff, and chill for 2- 3 weeks, while they grow out. Just foliar to keep their humidity up. Ime, worrying about runoff IF your plants are healthy is pointless. Runoff in the catch trey makes no difference in coco, accept for having to water more often.