I haven't heard the Threes yet, but I have heard the SB-3s and they already had a surprising amount of bass for such a small speaker. A sub is pretty much a necessity still though, long term. Yes, having the same drivers will make a small difference, like that last 10% towards achieving that ideal match, but you will likely be fine with the TwoC since it is still from the same family and should be fine timbre wise.
If you place a priority on surround, I'd get the surrounds first, then the sub, then the center. If you feel you need that impact, then I'd go sub, surrounds, center. A good 2.1 setup can carry you a long way.