What size tank are we talking?
Chances are you have male dwarf gouramies. Females aren't usualy offered for sale at LFSs and are much duller in color.
Having 3 male gouramies in one tank is asking for trouble - particularly when one is a betta (especialy if that betta happens to be a male as well). Idealy, bettas should not be mixed with any other gouramies.
Also, dwarf gouramies are quite fragile fish. The conflict that is likely to result from putting them in with a betta may even end up resulting in their deaths - if they are not directly killed by an attack (which is usualy sudden, typicaly over-night - bettas are unpredictable), the stress from being chased around or from competing over territory will get to them.
Back to your questions - as I said, your dwarfs are probably both male. You'd be hard pressed to get any fry from a spawn in a community tank even if they did happen to be a pair.
As for who amde the nest, there's now ay to tell unless one of the fish is actualy guarding it, in which case that's probably also the male that built it.
The fins you mentioned - these are modified, hair-like ventral fins used by the gouramies to 'taste' food and explore their environment. In the wild this makes them better adapted to living in murky waters. Normaly, each fish should have 2 of these. Your betta has them two but his/hers are thicker and a little shorter by comparison. They are the fins that come out of the body near the belly, before the anal fin. If either of the fish is missing one or more of these, he/she either was attacked and they were nipped off or had a serious case of finrot earlier on. Occasionaly they'll also come off by accident but this is rare. They grow back but may never be quite the same length and the experience is stressful for the fish so should be prevented from re-occuring. You need to keep an eye out for secondary infections occuring on the wound as well. These infections are usualy easy to treat but, with fragile fish like these, they can ultimately cause death.
