The Clarion XMD-3 is a Single Din. The face plate of the XMD-3 is a din and a half, So when you slide your Clarion out you will find a single din.
Im not sure how much your new HU will cover so you might have to make a face plate or not depending on what it looks like under there. The good news is it will side in and most of the work is done. The XMD-3 is held in by 4 bolts at the corners whe you open up the cd flap you will take off the plastic side covers that hide the bolts these will be where you might want to secure your head unit.