On the Display settings tab (Right click the desktop and go to Dislpay, the last tab) You'll see 3 monitors there with numbers.
-First you can click on the Identify monitors button, this will let you know with big numbers on the monitor which one it is.
-Then select the one that you want to be the primary, there's a checkbox that says This is my main monitor. Check it there and it will make it the primary, which means where the taskbar is.
If this doesn't work, usually the video card drivers have some sort of settings.
-Click on that same tab on the Advanced Setting, there you'll see a tab with something like NVdia or ATI or whoever is the card manufacturer.
-Once there look for the settings on the monitors and it should be something similar, some checkbox that says that you want that monitor as your default one.
Hope this helps.