How much stronger is a team of GMs compared to a single GM? For example Carlsen is currently the world number 1 and favourite to win a match against any other player in the world. Is he also favoured to win a match against Caruana + Mamedyarov + Ding Liren acting in consultation? How much stronger would a team of top GMs be compared to a single GM?

If it matters: standard classical time controls (shorter time controls makes it hard to consult teammates after all), the team of GMs are side-by-side, and they're playing electronically with the opponent in another room (so opponent cannot eavesdrop on their conversation). The team know each other and prepare in advance, and so does their opponent.
I think a team of GMs are not significantly stronger than a single GM.

My reasoning is: Thinking is fast while communicating is slow and error-prone. It takes dramatically more time to explain an assessment to another GM and while trying to explain some things will be omitted/forgetted. In the end a team of GMs will spend considerably more time to produce a quality move and that move may not be significantly better to justify spending extra time.

Plus, more teammates the team have, there will be more communication channels (grows quadratically respect to number of teammates) and this will make communication less and less efficient.

An alternative approach to overcome inefficiencies:  Let the team to be formed as 1 player (the best player) and N advisors advising player when they believe they can significantly improve players move, otherwise they don't interrupt. Ideally advisors should excel the best player in a particular area of chess. For example Adviser 1 is the best endgame player and advisor 2 is Sicilian expert and so on.

Also the team can switch player in the middle of the game to avoid player to get tired and play weaker especially in long games.