Center 3: join a pair
Step 1 / 3
U': Top face: turn 90°, front face goes right
The first two centers are on the left and right. The top has a pair in its back row; the front has a pair in its right column.
U' moves the top pair out of Rw's way, Rw brings the front pair up, U turns back.
Step 1 of 3: U'. Top face: turn 90°, front face goes right