So today I picked up a 2.5 gallon for my betta, Fiji, and on the instruction sheet it says this:
A As a 2.5 Gallon Aquarium: You can purchase (sold separately) an incandescent light and glass cover.
B As a Betta Aquarium: You can purchase glass partitions which divide the Aquarium into 3 sections. Each one holding a Betta. Then you can add a glass cover, which slows down evaporation and keeps the Bettas separated and an incandescent light so you can see their brilliant colors.
C As a reptile or small animal cage: For this purpose you may want the screen cover (sold separately). The screen cover locks into the Aquarium providing adequate ventilation and keeps you pet inside the Aquarium. And a reflector to keep reptiles warm.
B kinda makes me angry, I would keep one betta in these tanks.
But never three.