Question on task scheduling in Linux

So I need some help understanding this: Lets say I have a C program on
Linux that creates 3 threads of equal priority. At some point during
execution, one of the threads A needs to execute a special function
without getting interrupted by B or C (critical section within A). How
can this be done? Can scheduling of A, B and C (I assume it is round
robin) be disabled from within A? Or is is better to increase A's
priority? I am guessing disabling interrupts does not make sense on
Linux.
Please, any help,