proper mechanic question

[muxbule]

What is the correct mechanic when A1 tries a 3 pointer and is fouled by B1. The official would obviously put one arm out signaling a 3 point attempt then...here is where my question comes in. The foul is commited, do you just close your fist on the arm already extended OR switch arms to show you now have a foul call then go back up with both arms if the try goes in.