The Special Theory of Relativity deals with two observers, well... observing a single event, and comparing their observations. So:
You observe two spaceships, S-A and S-B, occupied by Obs-A and Obs-B moving towards you at $0.75c$ from opposite directions.
You observe S-A travelling towards you at $0.75c$; Obs-A observes you moving towards him at $0.75c$. Nothing strange here.
You turn in the opposite direction and observe S-B travelling towards you at $0.75c$; Obs-B observes you moving towards him at $0.75c$. Nothing strange here, either.
You conclude, correctly, that S-A and S-B are approaching each other at $1.5c$. If they started out 30 light-years apart, they'll meet in 20 years, right about where you're standing.
Obs-A will measure Obs-B to be approaching Obs-A at the velocity given in the answer above: $0.96c$. Note: MEASURE
The SPoR lets everyone in these three frames of reference predict what everyone else will observe, what their clocks will read at various events, and why various slow clocks and shortened meter-sticks make everything come out right in the end.