I had the same thing, I had one variable that is explained by 5 different subtle questions in a survey, to compute the variable I just ook the mean of the results of those questions. To do that I used compute, I guess you can do the same thing for the median. This is how it works:

1. Transform > Compute

2a. Fill out the new variable name under 'target variable'.

2b. Fill out the command under 'numeric expression'. e.g.

Mean(VAR1,VAR2,VAR3) probably in your case MEDIAN(VAR1,VAR2,VAR3)

3. Click 'OK'

Hope that works into the way you want it!