It's called an Eyefinity set up. One monitor must be connected to the DisplayPort of your graphics card; either natively (your monitor has a diplayport input) or through the use of a displayport adapter.
If you are already utilizing such a set up, check in Catalyst Control Center to see if there is a setting to enable Eyefinity.
Edit: Specifically which HD6750 did you purchase? Could you provide a link?
Active Display port adapters are at the very least 80$. They MUST come with USB or external power supply. 99% of the so called active adapters create latency or are actualy passive china brand junk.
Please correct me if I'm wrong, but my recent investigations into Eyefinity and 3D suggest that, if you have a 120Hz monitor, you will need dual link, if however, you're using a 60hz at 1080p then single link will suffice. So passive 3D on eyefinity, you use single, active (3d vision) you need dual.