Female bettas will absolutely NOT work. It's a good idea to avoid ever keeping bettas with other gouramies. All of them being similar fish, they tend to fight.
Also, don't get more than one male three-spot. Males will fight over the females and can even do so to the death.
IMO, tiger barbs are best avoided because they nip - not so much because the three-spots would be aggressive. However, aggression is also a factor but only if they were to breed once mature. Do you plan on letting them do so? If the tank is not heavily planted or there's a strong current, you shouldn't actualy have long-term issues with breeding aggression anyway as the bubblenests won't be built (and you can always destroy them as soon as you see them) and fry won't survive.
Anyway, a better option for schooling fish might be rosy barbs, golden barbs, spanner barbs, ember barbs, zebra barbs, clown barbs or one of the various ranibowfish species such as melanotaenia praecox or one of the slightly larger ones. Do avoid anything slow or that likes to spend most time at the top of the tank. Barbs are generaly an ideal choice because they tend to stick with the lower levels.
Be aware that, if you end up with a particularly aggressive male and he breeds, you could well end up not only with dead schooling fish, but with dead cories and dead female gouramies as well. Some are real devils and the only solution may be to replace him. Having said that, the more females you have, the less likely any male would be to go for other fish and certainly the less likely one of the females will turn up dead.
Obviously, the most simply solution to all the problems is to stick a female-only group! There are so many colors to choose from with three-spots, and the females still form hierarchies etc, so you don't actualy miss out on anything much provided breeding is not a concern.