Assume an alternate universe with same physical laws as here. In this universe nothing exists except 3 observers, each in a transparent box with a clock. Observer A is travelling at .5c. Observer C is travelling in the opposite direction at .5c. Observer B is at rest.
In the absence of outside landmarks each would feel they were at rest and their clocks were functioning normally. Suppose they pass each other. To A it would appear B had zoomed by at .5c and C at c. To B it would seem A and C had zoomed by at .5c in opposite directions.
Suppose they noted the functioning of each others' clocks as compared to their own in passing, and they met later to compare notes on their observations. What observations would they have concerning clock function? If C shone a light beam ahead of himself, how would it appear to A?