As asked by others, to what end? A, B, and C are arbitrary labels for Line 1, Line 2, and Line 3—which are arbitrary labels to some degree in and of themselves—representing the conventional connections further up line going back to the grid, the largest reference in the U.S. (eventually the power plants, but they all sync' to the grid).
Unless your POCO provides you with their distribution A, B, C, how the phases are connected to your service disconnecting means is all that matters for your site with the exception of rotation. To verify rotation, you either need a rotation meter which complies to NEMA standards, or a standard 3Ø NEMA motor, connected, bumped, and rotation verified (looking at end of output shaft, clockwise).
If rotation is CCW, you have to flop two lines, preferably B and C. Basically you want to keep POCO "A-phase" as your reference.