New to the forum and need some guidance!

[Oldfishlady]

I like snails and mine have a job in my little ecosystems, IMO a snail would be great in your 3g with a betta.

Look for products that are for R/O water, Seachem make a good one called Equalibrium.
IMO with a 3g tank it needs at least twice weekly 50% water changes with a filter to maintain quality water for the fish and fins and using bottled water could get costly. I have really hard water too and have found that all I have to do is tear down my filters and give them a good cleaning with vinegar to keep them running without issue and some of my filters are near 20 years old.