Option #3 - Divided aquariums
This option is third best because although it allows you to have more than one betta. However, this options has its flaws. For example, if one betta is sick, the rest will become sick since they share the same water. There will also be problems with the water flow. One betta will be swimming in a whirlpool, while the others won't have any filtration. And if the dividers aren't high enough, one of the bettas may jump to the other side and then you'd end up with injured or even dead bettas.
Option #4 - Betta Barracks
Betta barracks are similar in both pros and cons to the divided tank but is worse because most barracks are way too small. What it is is basically a smaller tank that either floats or attaches to the inside of a larger aquarium, thus saving space. However, because it is too small, the betta is more likely to jump out into the main tank and if the main tank is filled with incompatible fish such as tiger barbs, you can say bye to your betta.