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,