I posted a similar question in the thread "SR Questions". I suggest that you
take a look at the discussion there. The initial post is as follows:

We have three observers A, B and C in a straight line.
A----.9c----->B <------.9c------C
1. According to B, A is moving toward him at .9c
2. According to B, C is moving toward him at .9c.
3. According to B, A and C are approaching each other at 1.8c.
4. According to A, C is moving toward him at .994c.
5. According to C, A is moving toward him at .994c.

Question:
Why is B can see A and C approaching at each other at
1.8 c and yet A and C can only measure that they are
approaching each other at .994 c?

The solution:
1. A and C are indeed approaching each other at 1.8c
2. A's measurement of C's approaching speed of .994c is apparent and it is
due to that light is being transmitted by a medium at a max speed of c.
That's why A can only detect C's approaching speed to be less than c. A's
measurement can be corrected and making it equal to 1.8 c as follows:
      Corrected approaching speed of C= .994c(Fac/Faa)
Where Fac=frequency of a standard light source in C's frame as measured by A
            Faa=frequency of an identical light source in A's frame as measured by A

Hope this helps.

Ken Seto