An instant cast Fire 3 has a 2.5 sec recast, so you're simply replacing a fire 1 cast with a fire 3, from the perspective of averaged DPS. The only difference is the fire 3 is cast at the beginning, rather than the end of the 2.5s cycle. You still do two spells in 5s.

Quote Originally Posted by Hystify View Post
My suggestion for using Thunder only applies when you hardcast it, it should be understood that T3 should be used for thundercloud procs. Guess i'll edit the post a bit..
I don't think you understand the tool tip. Assuming it's not wrong for some reason: A proc generated by a thunder 1 cast will have 240 potency, regardless if you cast t1,2or3 for the proc.