The white-ish to clear coloring on the tips of their fins means new fin growth, not fin rot.
Ammonia build up you will see red streaks on his gills and head, along with gasping for air at the surface.. so doesn't sound like ammonia build up.
In a 3 gal filtered, 1 50% water only, and 1 50% substrate only water change per week is recommended. Either vacuume the substrate or mix it up in the tank and dip it out with a cup to get as much of the gook out as you can.
Clamped fins: holding his fins real close to his body and/or squeezing them tight like a folded umbrella.
I would stop the AQ salt, since it is not for a treatment at this time, it is doing more harm then good. You added in salt without the proper 100% daily water changes, which can harm because of the build up. Plus over time it will cause him to become immune to the beneficial properties of it so then it will become void when you actually need it.
You said yesterday you did his first water change, 10%, and when you listed water changes (in order) you done 2 more since yesterday? Did you add in the full amount of the conditioner per water change, and not the reduced amount to match the % of water that was changed?
Are there the appropriate heavy plants (real or fake) and hiding places for him? He may have found a place he feels safe at behind the heater, which some do. He is still new to the tank, so he could be still getting used to it and haven't explored yet.
Also, did you baffle your filter? If you had not, that could easily be why he stays in the corners because the current is too strong for him and stressing him out. A lot of bettas do not like, and do not do good with filters. Don't stop and start your filter, either keep it off and up the water change to 1 50% and 1 100% per week, or baffle it
It doesn't sound as if he is sick, but more like he is stressed.