The Fighter/Mage/Thief is an extremely versatile class though it suffers from dividing its' experience among 3 different classes causing level progression to be particularly slow.
As a Fighter it benefits from high Hit Dice, THAC0 and advanced weapon proficiencies as well as access to all kinds of armor and shields. This enables the character to act as a frontline melee character though it will never be as good as a pure fighter in that respect.
As a Mage the character has access to higher lore and mage spells although access to high level spells is only possible at the end of Baldur's Gate II. This means the player will require a thorough knowledge of the tactical applications of his spells if he is to act in that capacity. Also as a downside the character cannot use spells when equipped with armor (with a few exceptions).
The Thief grants acces to higher lore, thieving skills and backstabbing, enabling the character to act as an assassin for devastating damage, to detect and disarm traps, to hide in shadows in order to scout ahead of the group, to pick locks and to pickpocket. The thief's specific downside is the inability to use thieving skills while wearing armor above leather armor (again with a few exceptions).