Xara,
Like the others have said, even if the method you listed in your post isn't the optimal way some would do it, you still do have the right idea. In the end, it doesn't matter all that much which heaters go on which circuit so long as it is always designed so that each circuit is rated to handle whatever combination of heater loads you put on it at all times, and that each of these circuits are also properly protected within their maximum allowable current ratings. The only problem I have with your original proposal would be having the 14A load scheduling on any of these three 15A circuits. Although there are some exceptions in the NEC which allow certain fixed loads to be rated at 100% of the circuit protection rating, it is a good rule of thumb to always keep your max. full-load at or below 80% of that circuits current protection rating (in this case that would be 12 amps).
As one of the others mentioned; running 20A circuits for each of these rather than 15 might not be a bad practice in this case. Not only would it allow for future load expansion (i.e, if another small heater or two are later wanted), but this will allow you to stay well below 80% of the current protection rating, and would likely avoid a situation where nuisance tripping of your circuit breakers may be caused. Remember that although resistive heating elements have no where near the magnitude of in-rush current, that say a motor would have when starting up; heating elements always draw a little more current when first switched on cold (lower coil resistance), than their rating indicate (which are spec'd out for what the heater draws once thermal equilibrium has been reached in the coil following the brief heat-up period to its rated temperature). By all of this I'm simply saying that its possible that by designing your circuit to have a 14A full-load, you may end up causing brief overloading of the circuit, especially if multiple heaters were simultaneously starting cold. This of course wouldn't in any way be dangerous if your circuit was sized properly according to its rated protection, but like I said, could be a major nuisance if you start popping circuit breakers left and right.
In my honest opinion, I feel that 20A circuits are the best choice in this case. 30A circuits I personally feel would be overkill in your situation (unless of course you find that a circuit will end up requiring more than 16 amps of current capability (80% of 20A). A 30 amp circuit would also require upping the wire size to #10 AWG Copper at all points in the circuit, and aside from being more expensive, started entering the realm of "damn this wire is a pain in the ass to work with!". Anyway, Hope this helps.
