I see you've gotten some great advice.
Since you just moved him to the 3 gal tank, after he's been in his bowl for a long time. So it's possible that he's a little stressed out right now because he's in a different place, and there's a lot more room. (Change can be scary!) This alone, might be impacting his behavior right now.
I have four 3 gal tanks. I have filters, so I don't do anything with filtering or cycling. Instead, I do one 100% and one 50% water change per week. However, I've recently read some great threads about using a sponge filter, so I'm giving that option some thought right now. Waterdog wrote a good thread about sponge filters
Salt stays in the tank. It doesn't evaporate or get "used up." So, while you're using it, you only need to replace what you've removed (via water changes). For example: Say you're doing water changes in your 3 gal tank:
- You remove 1 gal of current tank water, so you need to replace it with 1 gal of newly treated water..... Add 1 teaspoon Aquarium salt into 1 gal of treated water.
- You remove 1.5 gal of current tank water (50% change), so you need to replace it with 1.5 gal of newly treated water..... Add 1.5 teaspoon Aquarium salt into the 1.5 gal of treated water.
- You remove 3 gal of current tank water (100% water change), so you need to replace it with 3 gal of newly treated water..... Add 3 teaspoon Aquarium salt into 3 gal of treated water.
Always make sure the salt is completely dissolved before adding it to the tank. And, as people have already mentioned, Bettas are sensitive to aquarium salt though, so you don't want to use it for more than 10 days.