When you update to 3.1, the system should first show your 5V5 Rank as 'Unranked'.
After your first 5V5 Rank match, your 5V5 Rank will visually update itself to its closest approximation calculated off an estimated 5V5 'true skill value' based on your performance in 5V5 Casual, 3v3 Ranked, and other modes. If your performance in these modes prior to 3.1 was awesome, you may shoot up to a high Rank immediately after your 1st match.
However, you may experience some 'odd' Rank movement as you go through more 5V5 Ranked matches. This is actually normal as the system is assessing your most accurate Rank based on your current performance.
This can also lead to different results for individual accounts as the early 5V5 Rank approximation can be different for everyone.
We give higher movement in the first 30 matches, so expect to see more movement than you have done previously in 3v3, both up and down. After enough 5V5 Ranked matches, the dust will start to settle, and your 5V5 Rank will find its sweet spot in terms of accuracy.
Keep playing 5V5 Rank matches to help the system assess your closest Rank, and you'll soon see a more consistent progress and less discrepancy with your 5V5 Rank compared to your prior placement in 3v3.