Also- I would get more than 3 female bettas if you were going to try this. Get at least 4, if not 5 or 6 (assuming you have room). This will spread out their aggression. Before getting more though, read up on betta sororities and setting them up- its usually a delicate process.
Then, so long as you have lots of plants for them to play in, and some caves (they like to 'claim' their own personal home) the school of betta girls should be okay with the discus- assuming the discus are ok with the bettas (I don't know much about the discus' disposition).
Just keep in mind, that nearly any betta keeper will tell you that bettas are a gamble. They are naturally more agressive than most community fish. And when they get grouchy, they get grouchy. So, you may find that down the road things have to be rearranged, or that the bettas were fighting for no apparent reason while you were sleeping one night, etc. But, this is the case with a lot of fish i suppose.
And you'll love seeing the males in the bigger tanks. They love to explore and play, and they really will use all of the space you provide them with. I had mine in a 55 community for a while, and he was just SO ecstatic (especially seeing as he moved their directly form a lil jar).