I believe that a character is supposed to get advantage when two or more of his allies are within melee range of a single opponent. So if Gobbo 1 has Lae'zel and Astarion in melee range, they are threatened, Lae'zel and Astarion are supposed to get advantage on attack rolls regardless of the goblin's facing. However, if only Lae'zel rushes up and comes behind the gobbo, she gets no advantage because the gobbo could easily spin around and face her because he is not threatened on any other side. If both are facing the gobbo, they get advantage because the gobbo is now having to divide his attention between two attackers.
Then, as Sneak Attack states, if a Rogue with Sneak Attack gains advantage on their dice roll, they can perform Sneak Attack against an enemy once per round. So only IF an enemy is unaware of a Rogue and/or the Rogue gains advantage on an attack roll are they able to use Sneak Attack. Thus, it would require either that the Rogue sneaks up behind the enemy and the enemy doesn't detect them or the Rogue attacks an enemy that is already in melee combat with one of the Rogue's allies.
This means that the player would need to strategically line up their forces so that maybe someone like Lae'zel is rushing forward to melee attack an enemy and THEN Astarion attacks that person. Thus, Astarion would get an advantage on his attack roll and Sneak Attack, making him strategically more valuable but also making Lae'zel more strategically valuable because you need both to rush up in order to acquire maximum efficiency. It isn't just Lae'zel rushing in and getting all the kills or Astarion, but both working in tandem. Thus, going with the spirit of the game, which is teamwork.
Nope, at least not according to base 5e rules. Flanking Advantage is an
optional rule variant, and one widely considered to be OP.
Rogues get sneak attack
damage if there is another ally within 5 feet of the enemy (and the rogue doesn't have disadvantage on the roll). Without that adjacent ally, the base rogue needs Advantage to get sneak attack.
But yes, your general point that working in tandem is encouraged in both 5e base rules (rogue gets sneak attack if an ally is adjacent to their enemy target) and 5e optional rules (Flanking Advantage) whereas in BG3 you get advantage (and thus sneak attack) for just walking behind or above an enemy. BG3 is using Flanking Advantage without the Flanking requirement.
