I know you're past the encounter now, but here's a few other ideas I was typing up:
If your Bard (Garrick) is level 3, I'm assuming the rest of your party is around the same level. Level 3 is a bit low to be taking on the bandit leaders -- they are around level 5 to 6. It's intended to be a difficult fight that requires liberal use of magic items, potions, spells and anything else at your disposal. Try gaining at least one or two more levels and make sure your party is well armed.
Consider going back to The Friendly Arm Inn, Beregost, Nashkel and any other towns you've visited - and talk to the townsfolk to find any side quests you may have missed (just talk to the people with names - you don't need to click every random "Farmer" or "Townsperson") . And check your journal to see if you've accepted any quests that you haven't started yet. You could also just strike out into the wilderness. Many of the more lucrative side quests are completely off the beaten path and can only be found if you go exploring outside the "critical path" of the main quest.
As far as tactics: know that ranged weapons are king in BG1. Each of your characters should start combat using some kind of ranged weapon (or a spell) - and you should try to keep your characters out of melee as much as possible, especially when you're low level. Difficult to do in the bandit tent since you're in a tight space, but out in the wilderness spread out and keep your distance. Be on the lookout for special ammo like Arrows of Fire or Arrows of Biting, and save those for the tougher fights. When facing groups of tougher enemies, order your entire party to focus-fire on a single target. Taking an enemy spell caster out of the fight early can make a big difference.
Your own spell casters should largely focus on defensive and debilitating spells early on. Magic Missile may look cool, but a single Sleep or Horror spell can potentially disable an entire group of enemies while Dispel Magic can shut down an enemy caster. You'll find wands of Magic Missile, Fireball, Lightning, etc. which can supplement your spell casters' offensive abilities while keeping their memorized spells reserved for defensive spells.
So, once you're ready to go back to the bandit leaders with a few more levels and a stock of magic items, a good tactic could be:
(1) Use all your defensive spells outside the tent right before you walk in so you're ready to go on the offensive as soon as the fight starts.
(2) When the battle begins, have your mage or bard cast Horror to disable at least a few of the fighters. The rest of your party should immediately unload with ranged attacks on the enemy mage (Venkt) to disrupt his spells and kill him as quickly as possible.
(3) If any of the enemy fighters weren't affected by Horror, have your mage/bard follow up with a spell like Blindness or Spook (or a second casting of Horror if there's more than one enemy that resisted the first attempt).
(4) Once the enemy mage has been eliminated, you'll want to focus on the hobgoblin Hakt since he uses poisoned arrows. Then take out the gnoll Britik, and finally the human archer Raemon. One potential tactic is to use Charm Person on Hakt, and then have him attack his own allies with his poison arrows.
Again, I know you're past the fight now - but you may want to consider leveling up with some side quests and trying the fight again later. In bypassing the fight, you're missing out on some good loot and experience -- and it sounds like you may be in need of both. If you're under leveled and under equipped, the next set of areas are going to get even more frustrating. I wouldn't considering entering the Cloakwood Forest until you're at least level 5 and have a good assortment of magic items. (Also... bring lots of Antidote Potions and level up Imoen's Detect Traps skill... a lot...)
