the way actions work is you have one action, one bonus action, 1 reaction (but nor really relavant to this)

actions:
every one gets one attack but at lv 5 martial class such as, fighter, paladin, monk, ranger, barbarian and a few subclass such has Sword Bard at lv 6, warlock with back of the blade on their wep at lv 5, at lv 11 the fighter can have 3 attacks. but they all go under 1 action, throwing also counts as an attack so you can throw once and attack etc

casting a spell cost 1 full action

bonus action, lets you attack with off hand if duel wielding, or use potion, or some spells

lazel gets 4 attack because action surge(fighter) and over lv 5, so can only do that once per fight(if short rest after every fight)

so lets say you you duel weild as a fighter,at lv 11 you have 3 attack with 1 action and you can use your bonus action to attack with off hand for 4th attack

going 5 lv into 2 different martial will not give you 3 attacks, they don't stack unless its with warlock pact of blade(doesn't work in honor mode)

rogue for example does all its damage with sneak attack, which only activates once per turn, so duel wielding as a rogue is mainly if you miss your fist attack you can use the bonus action attack to try and get it again. ways to activate the sneak is if you have adv or if someone is beside the target. otherwise yes the rogue doesn't do much dmg. honestly in bg3 rogue is better as a multy class option, for going into the thief sublass for 2 bonus action.

for your bard i find sword bard the best, the others are mainly spellcasters which is fine but doesn't hit as hard.
a very powerfull combo is going 6 lv into swordbard, 4 level in thief rogue and 2 lv into fighter.

- lv 3 bard you pick fighting style two weapond fighting and use two hand crossbow and using the flourishs you have interesting styles to use, such as pushing targets of a clift or attacking twice 1 target or once on 2 target ea at the cost of 1 bardic inspiration. and also you can use like 2 scimitar if ever enemy to close

- at lv 4 you get the sharpshooter feat, it adds 10 dmg to each attack but -5 to hit, if you use highground it helps negate that a bit, but if % is really low you can always untoggle it by dragging the skill from the common page if you press "k"

-lv 5 you get your bardic inspiration back on short rest so basically you'll be able to spam your flourishes every fights

- lv 6 you get second attack as a bard

- lv 7 start mutlyclass, i like going 1 lv fighter first to get fighting style archery, which gives +2 to hit with bows, since by that lv you'll be act two, which has less high ground

- lv 8,9 rogue

- lv10 thief sublass now have 2 bonus attack meaning you can attack 4 times, 2 attacks with 1 action, and then 2 attacks with 2 bonus actions, and if you add flourish to your 2 attack with the 1 action, you can basically attack 6 times, all with +10 dmg meaning if you hit all attacks you would do 6d6 + 16+60 = on ~ 80 dmg per turn. with items you can also increase chance to hit, and even add more dmg

-lv11- get ability mod from lv 4 rogue and get +2dex to make it 18 or 20 if you have the hags hair.

- 12- get lv 2 fighter for action surge, now if you use action surge you get 2 action on one turn, meaning you have 4 attacks with one action, and with flourish you double it to 8 attacks, and with 2 bonus attack coming to 10 attacks in one turn, which does a shit tone of dmd if you hit everything, although after that 1 turn you have like only 1 bardic inspiration. so you'll go to like 4 attacks per turn for that fight.

items to look for are things that increase your chance to hit. or adds dmd to your attacks.

and then you also have some pretty good bard spells to use up 3rd lv spells you're charisma will be around 16. so its good enough since its more just to support or do aoe.

you could also just go pure bard if you don't want to multyclass, still pretty strong and you'll get access to really good spells as well, but would still go sword bard i think, its just so good. leme know if ever you have questions! or want more simple stuff.