There are 3 lights in the form of a triangle. A, B, and C are lights and are stationary with respect to each other. S1, S2, S3 are spaceships.
A S3 C
The ships are moving clockwise. S1 is moving from A towards B. S2 is moving from B towards C. S3 is moving from C towards A. A, B, and C flash simultaneously in their frame of reference. So in the frame of reference of S3, A flashes first followed by C flashing. In the frame of reference of S2, C flashes first followed by B flashing. In the frame of reference of S1, B flashes first followed by A flashing. So the sequence of flashing is A, C, B, A. But wait! A flashed first. How can it flash last? How can A flash both first and last?