How this card actually work, i mean can this card declare example 3 allies and then all their defenses count together when you defense?

Yes you are correct but you could also commit more or less than three as well

And all damage should be assigned to only one of them.

Declare as many defenders as you want, Then add your total defense up with all defenders, then match it against what the total attack power is of attacker. If the attack power is greater than the total defense you just assign the remaining damage to one of the defenders. And of course if defense is greater than attack then there would be no damage taken. Hope that helps!