>>529356815
the rewards for the only mode that needs 3 teams are just 300 polys every 2 weeks
you also get that just by getting 6/9 stars, which means you can get away with only having 2 good teams at all, or having 3 mediocre teams
as for "raising a good roster asap"... I'm not sure what you're even asking, prioritize leveling to 55 on damage dealers and 50 on supports (you want to get everyone to 60 eventually but it's not necessary to start), leveling up passives, and putting a few levels into the character's more useful skills (5 or 6 is a good early stopping point). You also want to get at least some half-usable cope discs, something that has the right stat and is from the right set, generally discs are actually the most important thing to farm but it's completely RNG so it's better saved for last