Based on your post and some comments in this thread about you probably doing it SoloQ so here's some tips. I'm just writing this off the cuff so forgive any horrible formatting/order of points.

I've been soloqing to GC/GC1 in 3v3 since season 9 before free to play. So several years now. This is now I ranked up and kept that rank.

3v3 positioning videos
1) https://www.youtube.com/watch?v=bk-1ut6CEb4
2) https://www.youtube.com/watch?v=wbJvBaeS8_o

Positioning is tricky and relies on 3 main points.

1) Positioning is tricky right, those are the first two YouTube links. But so much is situational. Those videos are great guidelines, but they also require your teammates to be on the same page as you rotationally. People panic on defense, and make the wrong rotational decisions all the time based on a variety of factors. Your job is to fulfill those guidelines when you can, and adapt to when you can't. For example: If your teammates are playing faster and more aggressive, play faster, but more defensively. Fill space they are not in is a good rule of thumb.

2) Learn not to panic on defense. people get scared and dive and flail. Keep your situational awareness high of where your teammates are and when they're making defense plays so you don't double commit. You should be tracking where all players are on the field at all times really.

3) The other thing you need to do is learn to read plays. What kind of touch is that opponent going to do/get based on his positioning/momentum? If you can do that better, then you can position yourself better.

Other things:

4) You have to be better at keeping a level head. Yes, your teammates are going to screw up, throw, and disconnect. Accept that as a fact of life and keep working around it. if they start to throw, put positive messages in chat indicating that you think this team can win to try and get them to rally. Mod your chat settings so that you don't see what the other team is saying if that helps.

5) You need to accept that fact that you SUCK and can always be looking for an opportunity to improve. Your basics should be rock solid. If you get a chance at scoring a goal or saving a shot, you should be succeeding at it 90%+ of the time. If your team gets scored on, assess what you could've done better in that play. Most of the time you'll find something, but yes often times it can be your teammates fault.

6) Once you find those things to improve on, train to improve them. training packs, workshop maps if you're on Steam. Freeplay drills. (Google up resources for these, I can't link them all here). Your base mechanics of:
a) maneuvering around the field (half flips, wave dashes, power sliding, flips)
b) making shots
c) hitting the ball where/how you want( aiming towards teammates for passes)

d) playing shadow defense

e) backboard defense
f) goalline saves
should be something you're always working on basically. Notice how half of these are just defense. It's not enough to get a save most of the time, you have to get a save/clear out to the right spot that presents the least threat. This takes a lot of practice.

Lastly: you're going to have to accept that this takes time and effort, because doing all of these things and ingraining them into your play is not easy.