If you don't want a shrimp think about a snail or two. They need to eat veggies as well as algae, but they do help with that. Three gallons is too small for anything else. You could TRY an ADF if you have a lot of hiding places, but be ready to change the water every two days and be prepared to separate them.
I have a 16 gallon tank with three female bettas, 2 honey gouramis and five cory cats. That's more than enough. It has three different kinds of live plants and wood and caves, a Fluval chi grass pad that the cory cats love, and I STILL change the water once a week and have to stay on top of it so the tank doesn't get like, toxic.
On the flip side, in my five gallon I have one cory and a male betta, again with live plants and a cave fixture. They all have filters so that helps.
Do you have a filter for your three gallon?