Welcome to the community! There are many options for a tank of 3 feet! Try visiting the
lfs several times, and seeing which ones are the most interesting. Research them and use that info the next visit to the
lfs.
Fish can require different water types, pH, temp, hardness or softness, and tank inhabitants can need groups or need to be kept by themselves, or pairs or trios, lol.
Find your favorite fish types and build the tank around them!!!
Some like rocky substrate, some sand. Lower light, slower or fast flow of water.
Plants and cover or hideouts in rocks.
Many fabulous choices!!!
Check out the link in my signature for an article of all the basics and some great pointers!