5e rules don't actually have flanking at all, but some form of house rules for flanking is pretty common. The most common I've seen is that flanking attackers get advantage, which sometimes feels a little too strong. I play in some games where flanking attackers get +1 or +2 on their attack rolls, which feels a lot more reasonable. Advantage is a pretty big deal to give away that easily (not that it's always easy).
It’s a variant rule in the DMs Guide. I think it depends. In TT I wouldn’t give advantage if two guys were standing right next to each other. But opposite sides? I can see that.
Three guys surrounding you? Absolutely.
In the DMs Guide you have to be on the opposite sode to have advantage. That make sense but I guess it's not gonna solve the kangaroo problem.
The best solution should probably be :
- opposite side - and jumping rework -> bonus action if they want but not possible while engaged + not mixed with disengage. I could only have said "not mixed with disengage" but seriously, I can't see those jump at every turn anymore.
That's terrible in such a game and they have to find something to avoid players from jumping that much in combats. It looks totally ridiculous.