You aren't forced to target the same target each time... but if you do you get an extra D3 to on the 2nd and 3rd spells. Means you would be getting 2D6+D3 while only using 1PD from the pool.
The dragon mage is awesome... shame he's so schizophrenic: having a non-combat mage on a dragon is a little annoying, especially since a few bow shots will kill the mage. Dragon mages probably arent good for really competetive lists... but if all you want is a bit of fun look no further.