You'll probably find once you have Thunder 3 that its casting time is too long to properly squeeze it in when you have Umbral Ice 3 up (that is, you've just exhausted your MP for Fires, and you've just casted Blizzard 3 to start restoring your MP). It'll take two ticks for it to restore your MP back to full, but you'll usually need to wait for the first tick just to be able to start casting Thunder 3... and you'll get your second tick before it finishes, leaving you with less than full MP for shifting back to Astral Fire (unless you wait for a 3rd tick, which isn't generally recommended).
You have at least a couple of ways around this:
1) Swiftcast the Thunder 3 (or if Thundercloud has gotten a proc, obviously that works too). Just note that at 50, you'll probably want to use Swiftcast for Flare, and instead...
2) Cast Thunder 2 at this point instead of Thunder 3; if you start casting immediately as you get your first tick it should fire just before you get your second one. Obviously, if you do this, you'll still want to keep Thunder 3 handy for Thundercloud procs.