I've thought about it (generally two rather than three) but this doesn't work for the NR in the same way as it works for the IR for a few reasons. With the Empire, when it was under different leaders it had very different military and political focuses and doctrines which we tried to reflect with the subfactions. With the New Republic, even under different leaders, the overall focuses of the faction were the same. After a while, it's also difficult to make and balance that many different playstyles without having too much overlap. It effectively becomes just about having one or two different ships for each one, which is less than ideal for a few reasons. For one, balance in Sins is a bit more strict than in EaW. Generally you're trying to geta bit more out of each ship slot. Secondly, the ship slot counts are hardcoded- we can't add more pages of ship build options, so focusing on different ship options doesn't really work out, and then we need to use a lot more art resources on a single faction, which is important not just for development, but in-game as well- Sins has some strict limits on memory, so using that for extra assets within the same faction when we could devote them to another faction with a more distinct playstyle would be less than ideal.
As for heroes, unless we put in multiple structures to provide the heroes, we can't have that many heroes. With the Empire, it works because between all 3 subfactions, you only have 5 total heroes, and you can only have five total ability slots. With 3 different heroes for each NR subfaction, you'd need at least two structures for it, and that would generally get a bit confusing for players.
There's an additional technicaly limitation in that getting all that research in, if you can have enough to differentiate it, is you basically have to replace an existing page with it. With the Empire, Diplomacy is luckily a small part of what they do, so we could co-opt that page for it. With the NR, there's less opportunity for that.
The main thing, though, really is in the balancing and the diversity- the diversity within the NR just isn't there in the way it is with the Imperial differences, where they focus on superships vs ships of the line, research to improve their ships vs research to spam their ships, etc. There's only so many different playstyles you can have within a game, too, especially if you want it to be balanced, and we don't want to spend all of them on the same factions. We have four factions as it is, and we feel we've generally succeeded in giving them differing playstyles, 6 overall if you include the Imperial subfactions. We feel like we also have decently unique playstyles planned for the other upcomign three factions, bringing us up to 9, but the more we try to add subfactions, the more that gets diluted, and the more difficult it becomes to try to balance around those differences, especially for something so heavily cfocused on MP. It's really easy to get to a point where only one is seen as competitively viable (like Daala is now).