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Hey!I had the same impression at first glance but playing a FMT changed my mind completely. It might seem like FMT can't compare to M in terms of casting and you would be right about that. However, FT is among the strongest melee/utility classes in the game and adding mage provides a huge amount of extra survivability (stoneskin, mirror image, etc) as well as freeing up a fair number of thief points (because invisibility can substitute for your two stealth skills).
Fighter/Mage/Thief is an odd choice of character, imho. From what I've researched, F/M/T is a versatile, powerful multiclass early on, but falls short in the mid to long run. It takes too long to have a decent amount of spells to cast, meaning you won't be a very good mage for most of the early and mid content. You miss on the better upper level spells, too. Maybe I'm not seeing it for what it really is, but to me F/M/T is not a very good choice of character.