When rotated in increments of 90°, each 90 degrees represent 1 turn, or a movement 1 quadrant forward or backwards. Rotation is a turn forward or backwards about a certain point.
Reflection is a mirror image over a line of reflection. Translation is a slide left or right, up or down. These are RIGID transformations, which means the size and shape will NOT change, just the location of the point, line, or figure.
Since your direction is to use a different letter for each, I will suggest using M for reflection since they make mirror images (normally it is a lower case r). There are actually standard letters used as symbols for transformations in math.
