With what I've seen on XP for kills and the interaction of spells vs. opponents, I wouldn't.
(..if you just want that 3 party grouping then I'd still pull from from the characters already provided, not much has changed on that issue IMO. Even spell selection should be about the same.)
Once I've thought about this more I'll start a new thread, but the gist of it is this:
You'll get more XP throughout the game by level-squatting AND introducing new characters in a staggered arrangement.
Once I determined that the party was sufficient to handle chapter 2's Battle Squares - I stopped leveling, and instead just inserted new characters at different points (though leveling-up those new characters as the game progressed).
The "kicker" to this is that once you get to about the middle of chapter 5 that you can level-up all those levels on *1* character and still get rather excellent XP for kills.
To give you a better idea (with some "harder" numbers):
A party of 6 started at the beginning with continued level-ups can achieve characters somewhere between level 16 & 17 at the very end.
I'm in the 2nd level of Dragon's Eye (chapter 5) and I've got:
one character that could be leveled to 21, one character that could be leveled to 20, two characters that could be leveled to 19, one character that could be leveled to 13, and one character that is level 7.. and I use them ALL.
They are actually:
One level 19 Cleric/Monk, one level 11 Wizard/Fighter/Barbarian, two level 10 Sorcerers, one level 10 Druid/Cleric, and one level 7 Paladin/Fighter/Rogue.
I should note that where I stopped on the leveling process, wasn't optimum for party effectiveness. (..and it's something I'll discuss in more detail in a that new thread.)