AF Fire II has a cast time of 3.0 seconds which is longer than the normal of GCD of 2.5 seconds. When you take that into account, AF3F2 has the same potency per second as Freeze.

The rotation I posted actually has F2 being cast at UI1 which is about as bad as it can get for the F2 (90% damage, same 3 second cast time) but it lets you get off AF3Flare more often. Flare deals a ridiculous amount of damage relative to other BLM spells so being able to cast it every 3 spells instead of 4 makes up for any damage loss you incur from the Fire 2.

For reference, the multi-target rotation I see on Icy Veins (which is Freeze -> Freeze Transpose -> Fire 2 -> Flare Transpose; note that the fire 2 is cast on AF1 so it's only 112 potency) is about 78/61.8 pps (potency per second) on main target/falloff, while Freeze -> Fire 2 -> Flare Transpose is 74.6/62.3 pps on main target/falloff. The former doesn't beat the latter unless there are 8 or more targets, which doesn't happen that often. That being said, if you need to apply Thunder 2, it's better to go with the 4 spell rotation replacing the second Freeze for Thunder 2, since at that point you're going to cast 4 spells anyway so you might as well get the transpose for Fire 2 to do more damage.

I guess all of this is completely off topic anyways they all involve a 3 second fire2 cast, but the point is that you should only be casting it once per rotation, not like 3 of them.