Welcome to the site.
I am sorry to have to say this, but a 20g tank is unfortunately not a suitable home for a common pleco. This fish can reach close to 2 feet. It will not stay 3 inches for long, and it would be better for the fish to be in a much bigger tank.
As for fish that can go in a 20g tank, there are hundreds. I actually suggest you try going to your local fish or pet store and looking at what they have in stock. Write down the names of fish you think are cool, and then research them at home.
For a few ideas, some random tropical fish that will be ok in a 20g tank include neons, black neons, scissor-tail rasboras, harlequin rasboras, Pristella tetras, serpae tetras, black phantom tetras, Bolivian rams, clown pleco, bristlenose pleco, dwarf gourami, and panda cories.