I noticed that it is very quick to finish the submit indeed. The first 10-15 percents seems to account for about half of the total number of games you play to complete the submit. I suspect that there is just simply not enough game in regard to the ranking algorithm.
I see you managed to rank 2 now after a few submit ending up in between rank 5-16 that is a lot of variance, it's pretty hard to work on improvement with such an unstable ranking
Regarding those 3 players game I also feel they are quite unbalanced. If in the long run it should evens out, they are indeed more variance prone because of placement. Depending on the breakable box positions it can lead to 2 players struggling vs each other on one side while the remaining player kind of peacefully collect most of the other half, only entering the struggle in the late game.