Seedling burned?

hard to tell from picture, but light burn would show up on the leaf edges, looks more like overwatering. Most plants don't like wet feet. There's always a happy medium not too dry and not to moist.

Adding circulation fans could help. But it also looks like that seedling is in way too big of a container. Even if you didn't over water the shear amount of water it took to soak the medium is taking too long to dry and the plant can't uptake all that water. That plant should be in a seedling sized root-cube tray or like a 2-inch little seedling pot.

 

I'm not too versed in autos i've always grown photo-period, so i can't speak for autos. I can though talk about plants and transplanting from root-cube size is typically a very common practice. And in most books i've read they have also said this.

Maybe an experienced grower can grow directly in a 3-gal but I would establish the plant first in a root-cube or 4-inch pot then from there you can go to a 3 gal after a few weeks.

I do this and have had very little issues with seedlings but before the 3-gal I like to put them in a 2-gal

 

Autos don't like stress, they ony have a set number of days to live, so anything that slows them will remove from your yield. You did right starting in the pot you want to finish in.