You really shouldn't swiftcast Fire3 while in umbral. You are already constrained by the GCD, and you lose your ability to sneak in an extra Fire before the Astral buff kicks in.

One other note: as above poster points out, one T3 while under Umbral 3 is usually enough time to regen to full. But if you time it correctly, you can fire off Fire3 and recover full mana before it lands. Not much, but somes enough to add another fire at the end of your rotation, or immediate T3 after switching to Umbral.