1 gallon of water is not enough to provide each betta with enough swimming space. Bettas need at least 5 gallons of water per betta.
In a gallon tank, the water would also become polluted very quickly, which could lead to health problems for the bettas.
Female bettas can be aggressive towards each other and can injure or even kill each other if they are not given enough space.
Keeping 3 female bettas in a gallon tank would lead to overcrowding, stress, and ultimately an unhealthy and potentially deadly environment for the fish.