OK, I have a couple 3gal tanks, you have a few options.
1 Betta with a couple african dwarf frogs. The don't move much, but they will be fine with a betta.
2. You can get a divider for the tank and get a 2nd betta male or a betta female.
Note, the female needs to be seperated from the male at all times unless you plan to breed about 200-300 babies! Both the males and females can jump over, you need to make sure the divider goes high enough to prevent jumping or has a plastic cover.
Yes, bettas will die if they get too cold. Below 65F will kill him fairly quickly.
Most petstore bettas live 1-2 years after you get them. Most are between 8months to over a year old in stores when you buy them. There are some stories of 3 to 4 year bettas, but that is rare.
To keep a betta long, you need to slighly UNDER feed him (too much food leads to higher metabolisim and more waste), keep his tank temp between 74F and 78F as much as possible, and have frequent water changes.
I would personally prefer to have 1 betta male in a 3gal with a nice slow filter, some floating live plants like hornwart or javamoss. Maybe a few taller low-light plants like javafern or watersprite.