A 10G divided 3-ways is a delight to keep. Finding the right filter can be tricky, but once you get that down you're golden.
Filter is a Fluval U1, which is VERY quiet and directs a steady flow of water along the back, allowing circulation even in the last section. Choose the fish that is least affected by currents to be in the sector with the filter.
I've tried "roomies" with them before, the results were less than desirable. As in, dinner was had. Bettas are territorial and food-oriented. Snails are a better choice than shrimp in my opinion, but I hear they poop a lot so...
I'm going to be putting up a tutorial in a day or so on how I make my dividers, if you need any help with that. There's a lot of DIY methods out there, but some are flimsier than others.