thats not a very good strategy because mage has weak fire power and archer has waaaay stronger fire power and warrior is just melee attack with the biggest strength and defense. So the warrior should go with the mage and the archer alone
That's true IF the archer is a high level. With a low leveled archer, the mage should go alone. Your acting as if the mage is defenseless. If they put points into Support Stick and Healing, they should have absolutely NO TROUBLE going alone. Same basic idea for the archer, and the warrior is the least important of the three because they die most often due to charging into the hordes. Even with the mage, he would just be wasting the mage's mana. The mage would have to be good at multitasking to both attack, heal and defend at the same time.
So to sum it up:
If the archer is a high level he should go alone. If the archer is a low level the warrior should join him. The mage should put points into Support Stick and Heal and will have no trouble defending himself. Even if he does get overwhelmed, he can just slowly fall back while attacking and the enemies will usually be dead before they get to the castle.