The goal is to create a secret S on a trusted server by having users A, B and C each send a private seed to the server, in such a way that any 2 of them can later recover S by resubmitting their corresponding seeds.
Edit: the actual use case is to create a secret that the server can use in memory but won't persist on the disk. If the memory is reset, any two users can collaboratively recreate the same secret in the server's memory (with the possible help of some persistent recovery parameters that cannot, on their own, recreate the secret).
It looks like a "reversed" version of the Shamir Secret Sharing Scheme, where all users start with their chosen share and the algorithm needs to crunch those shares into a secret that meets the threshold recovery criteria.
What is the simplest way to implement such a protocol?