I Honestly don't see a problem diving it 3 ways as long as there even. That will give you a little over 2 gallons per fish which is plenty (& Yes I am well aware that substrate removes from that). Especially given the shape which will still provide plenty of surface area to each one. The other things to be sure of is that there is filtration & heat working for all 3 sections. The dividers themselves will act as baffles so if you have a HOB filter in the center it will have LOTS of flow & very little in the outside 2.
This said If you go that far as to add 3 betta's keep anything else VERY light in bioload.. So I wouldnt add ANY snails, though a few good sized shrimp would still be OK.
Although if it was me it would still start out as just being heavily planted for a few weeks. Followed by adding a shrimp colony. Lastly adding a single male. Although I think 2 gallons is enough room as long as the water is taken care of I still prefer to give them more. & The Petco Bookshelf is such an awesome tank to show off in I would do just that.