The problem is less one of classes and more one of roles.
If the party was playing full-on Wizards, it could essentially accomplish anything it wants through a mixture of Battlefield Control, Crowd Control, Damage Spells, and Save or Die Spells. This is because Wizards are incredibly versatile and can cover many different roles.
In general, your party will find itself in need of:
- Battlefield Control: the ability to remodel the battlefield to your advantage, such as denying access to certain areas, creating cover, etc...
- Crowd Control: the ability to temporarily take some foes out of the fight, such as putting them to sleep, sending them sprawling, grappling them, etc...
- Diplomacy: the ability to deceive, persuade, and in general obtain what you want of others.
- Healing: the ability to restore characters to 100%, also includes removing status effects, such as Blind/Deaf, curing Diseases, restoring Ability points, etc...
- Knowledge: the ability to know what you face, its strengths and weaknesses; generally necessary for proper planning.
- Stealth: the ability to move in/out undetected, because sometimes battling the full castle guard is a bit too difficult and counterproductive.
The lack of one or two capabilities can be challenging and interesting, the lack of all such capabilities simply makes a number of encounters nigh impossible.
Furthermore, all the party members will have the same weakness. A well placed Fear spell, and the whole party is running around, with nobody covering the others.
I would propose that you show your party a number of varied typical encounters, and explore with them how the party could possibly overcome the encounter, at various levels. If they are inventive and manage to surprise you and solve the problems, it may work. If they cannot figure out a way for over half the encounters, they are doomed.
The Same Game Test1 is a pretty useful resource for this; it "benchmarks" a character or party at level 5th, 10th, and 15th.
Let's start with level 5th:
- A locked door behind an arbitrarily high number of assorted CR 4 traps.
- A huge Animated Iron Statue in a throne room.
- A Basilisk in its desert burrow.
- A Large Fire Elemental in a mystic forge.
- A Manticore on the wing above a plain.
- A Phase Spider anywhere. They're tricky creatures like that.
- A couple of Centaur Archers in a light to medium wood.
- A Howler/Allip tag team in an abandoned temple to a dark god.
- A Grimlock assault team (4 members) hidden in a cavern.
- A Cleric of Hextor (with his dozen zombies) in a crypt.
I can see the party handling itself well against the Animated Iron Statue, Large Fire Elemental, Manticore, Centaur Archers, and Grimlock assault team.
How do they plan to deal with:
- Locked doors/coffers? Always breaking them?
- Traps? They won't even see them.
- Basilisk: one failed save (Fort. DC 13), and one team member is Petrified.
- Phase Spider: how do they reach the Ethereal Plane? Handle the Poison (Fort. DC 17) and subsequent Con damage?
- Allip: incorporeal, and drains Wisdom (1d4 at a time). If they cannot reach it (Incorporeal) they'll become helpless when their Wisdom reaches 0, soon to be dead.
- Cleric: it'll be a tough fight, as the Cleric should be using Battlefield Control/Crowd Control against the party.
From the little information you gave, your party is more or less doomed. It can handle straight fights well, and that's it. Anything else, and it's a sitting duck.
Show them this test, show them how woefully under-prepared they'll be. Don't coddle them, show them how harsh reality can be straight from the first level, when they still have low-investment in their character. And be accommodating if they wish to switch.
1 See this answer by Hey I Can Chan for more links to the Same Game Test, including links to already run tests.