Playing 3v3 with a two-player party as your teammates is the worst.
I'm sure I'm not the first to say this, but it's such a frustration and I've always wondered why it's allowed, especially since the Solo Standard/3v3 playlist was rid of years ago.
Far more often than not, the two partied players on will play as though the third solo player isn't even there. It makes you wonder why they're bothering to play threes.
As the third leg, your best bet is to usually just back them up and play conservatively/defensively the whole time, because those two will just go for most every shot/challenge without considering the third player may be in a better position to do so. And that's just a really simultaneously boring and aggravating way to play.
I've long wanted (hopelessly) for the Solo Standard/3v3 playlist to be resurrected, but it's not because I don't like playing with randos against partied three-player teams, it's because I cannot stand playing on the same team with two partied players.
Between allowing this and getting rid of Solo Standard I sometimes feel as though this game is actively hostile toward people that enjoy 3v3 but don't want to be in a party. I know that's an overreaction but damn, it kills my enjoyment of the game a fair bit.