How does the 3v3 ranking system work?

So me and my ranked team played our 5 3v3 matches and we won the first 4 and lost the last one. The first four teams we beat were silver 2,3, and 4's in ranked 3v3 and we slaughtered them. Our average score for that was we had 20 kills they had 5. Now the last one we lost to a team that was also still in their provisional and was 3 for 3. The score was about even until the last fight. We lost the game but we were expecting to at least be placed in silver 5, the least, but instead we were put into bronze 1. I understand that we cant just be thrown into silver and so on, but how does that work? Can someone please explain to me. (This also happened on the last team we had made where we won the first 4 and lost the last one and we got placed into bronze 4.)