Current in Secondary Windings of Toroidal Transformer
is "Is" in that formula just the maximum available current and the
  motor will draw what it needs?


The motors will draw what they need - just because you take 30 amps in one winding it doesn't mean other similar windings are forced to deliver 30 amps.

You will see some "regulation" issues but these may not be significant. If you draw 2160 VA for the three big motors you will see the 15 volt secondaries lower there supply maybe 5 or 10%. It's inevitable because the primary winding isn't perfect and it has losses - you take current from a secondary and this gets transferred to a current on the primary and, leakage inductance and copper losses means you lose some ability to keep delivering voltage perfectly to each secondary.