Wasn't it already confirmed that they'd all be 3 Vs. 3?There's no confirmation that the Masters 8 tournament battles will be at most 3v3s, so I don't know why you're jumping to conclusions. Leon vs Lance was only a 1v1 because the writers didn't want to throw a full 6v6 battle at us so early on into the series. However, the battles have been getting progressively larger as we move up the tiers, with 3v3s being introduced in Hyper Class. Following this trend, it's likely that we'll start to see 6v6s in Master Class, just like how most league battles end up as 6v6s by the quarterfinals (as you've said).
Also a tournament that only has you fight 3 people is so very BW of Journies.