Multiparty secret generation protocol
You can do the following:


The server generates a linear equation $y=ax+s$ where $a$ is chosen randomly from a finite field $F_p$ and $s\in F_p$ is the secret.
For the values from $A,B,C$, let’s call them $x_A,x_B,x_C$. Evaluate the linear equation at these $x$ values and the server gets $y_A,y_B,y_C$.
The server returns $y_A, y_B,y_C$ to $A,B,C$ respectively.


Each party now has a share which is a pair $(x_i,y_i)$. Any two shares can reconstruct the secret $s$.

This is essentially (2,3)-Shamir secret sharing.