Why the F/M/T combo doesn't get the special abilities for mage, only for fighter and tief?

Because it is a bug.

Mage high level abilities are divided into two parts: one part has abilities that require a char to be able to cast 9th level spells, and the other part (extra 6th, 7th, 8th spell memorized) doesn't require 9th level spells.

Since you have only 2,666,666 max XP for your mage class, you cannot get access to the first part of abilites. The other part has been simply forgotten. Somewhere (SP or Baldurdash) there is a fix for that.

On Baldursdash.

You can get the Triple-Class Mage High Level Abilities Fix at Baldurdash (http://members.rogers.com/mrkevvy/ToB/Fixes/TOBfixes.html) . ;)