I've done a bit of digital zoom on this photo so you can see three distinct rocket motors burning. I believe that it's the two SRBs on the outside and the three SSMEs blurring together in the middle, but the SSMEs themselves also have a delta configuration, so it's possible it could just be SSMEs and no SRBs. Any experts out there are welcome to send corrections in email.
Copyright 2011, Garrett Wollman. All rights reserved. Photograph taken 2011-02-24.