I think right now I like Doug Hogg's suggestion best.
Using numbered rows allows you to use hand signals (one, two, or three fingers) for the number of the row, and point your thumb left, up, or right for position.
3 left . . . . . 3 center . . . . . 3 right
2 left . . . . . 2 center . . . . . 2 right
1 left . . . . . 1 center . . . . . 1 right