there are also stitching functions ect... but they don't always do a good job. I would rather do it manually.
It's a tad more complicated than simply dragging the three pieces over to the same canvas though. You'll need to use the transform tools to rotate it slightly so that it matches up just perfectly. Simply right click on the piece you wish to alter and select that layer. You can drag the layer by simply, well dragging it. If it needs rotated in the file menu it's under edit/transform/rotate.
Hope that helps.