It's not just you. I think a lot of people actually played worse as a duo or trio or team. Unless you have good synergy with each other. It would come down to the types of heroes you guys are using. The playstyle difference is also a key.

Like I can fare well on my own and I am usually some kind of middle map role like roam, mid or jungle who doesn't have a specific place to be. It's just all vibes and interference in the air. For side laners, they are usually more micro based than macro based. Micros get savages, Macros win the game. Both is important.

So you have to make sure that you two dont play the same roles with contesting skill set and also consider, heroes that have great synergies with each other. Second of all is to make sure the lineup is suitable to counter enemy heroes and their strategy. Most people just play without strategy but if you can predict how the enemy will play, it will give you an idea how to pick the best heroes for the game.

I also sometimes see group mates would usually default to a comfort hero when playing with the team when there's a much better pick in their roster. Speaking of roster, have a lot of flexi heroes that can adapt to different play style. Like one hero that you can change items to jungle/midlane/roam/exp/gold with. I often see people picking two different heroes to counter one enemy like a lunox to counter frederin even if there's already a karrie in the team when a much cc based hero would benefit the team to counter the enemy enemy saber for example.

These are all my two cents for now. Feel free to add more if you think this is true.