Careful there, since what you said isn't how it works. Flex queue is entirely separate from solo/duo queue, so no mixing.
In solo/duo, you will only play with/against solos and duos. There may be arguments that it should be pure solo, but as it's stands it's both solos and duos, but you'll never face trios/quads/5mans.
In flex, it's intended to be a more organized team environment, so by queueing solo you're counting on your ability to mesh with randoms, so facing against premades is somewhat to be expected there. If you want a more individual thing, then that's what solo/duo is for.
In both cases though, matchmaking does try to match against premades of the same size. So if you're play flex as a trio, you will most likely face another trio. When matchmaking fails to find a like sized premade, it does give some behind the scenes mmr tweaks to account for the premade advantage, so while they might be a bigger premade, your group in theoretically slightly more skilled so the game is expected to be 50:50 based on the mmrs of those involved.
It's possible when you said 'flex' you really just meant a premade, and not the actual flex ranked queue and you knew they were separate, but probably worth pointing it out to be sure.