"Buy six to get a pair" is the age old concept... This will give you an extremely good chances of having both males and females... but if they pair up, that's simply up to the fish.
In my experinece Oscars start showing an interest in mating a bit later than most other Cichlids. But my experinece in creating pairs of Oscars is quite limited. I will say 8" is normal/common, but smaller is definately possible.
Peronsally I do not do the 'buy a group to form a pair' approach, as I like to take extra steps to avoid sibling pairs, but that's just my thing. Many others will agrue that this is unnecessary and even at times a good thing. I don't refute that, but simply share at other times it's a bad thing.
It's possible for a single pair of Oscars to produce all three morphs you mentioned if their genetics are lined up just right... so it's possible yours are siblings, but I'm not sure what the typical Oscar breeder/supplier does to make morphs, and I'm sure different people use different approaches. So they also many not be closely related.
In the long run I believe it is extremely unlikely that all of these fish will get along in your 125, but it doesn't sound like that is your plan, so no worries there.
Good luck!
hope all goes well