Lichess has a Play With The Computer option on its main site, which offers Stockfish levels 1 through 8.
What would their FIDE ELOs be with time control set to 5+3?
(Some people on Lichess have FIDE ratings, so if they play Stockfish at different levels often enough, perhaps one could use those games to estimate the different Stockfish levels' playing strengths. And perhaps this has already been done?)
The Stockfish different levels relate to how many moves ahead Stockfish is premitted to consider when choosing its move. Stockfish does not make the types of errors that most people make, so on a lower rating Scockfish will lose to good players in different ways to how a human would.
But you are asking how do use this information to predict "human to human" play....
However their are machine learning based chess engines that been trained on humans at different rating levels, so will play somewhat like a human at the target rating level.
See Why not use computers to evaluate strength of players? and How to make an AI chess engine which can play like a human of similar rating?
Assuming you play one of them at 5+3 time controls and are as likely to lose as you are to win, then how good would you be on FIDE's rating scale?
Ratings are not absolute values of chess ability or play. They are relative measures within a certain pool of players at a given time period. Hence there is no possibility of making such a comparison with any degree of accuracy.
According to Lichess, their site rating would be 3000. At the time of writing this answer, no user has a rating above that. I'm not sure we can give an exact number but it's definitely above that of the average Grandmaster.
