hai, the arrows disapear completely, then one can be  seen (the one in his shoulder), then finally two again. When only  the one arrow is seen, you can still see the blood stain from the  second arrow, but the arrow is not there. When he is hit by the  third arrow, all three arrows are visible. When Aragorn is kneeling  over Boromir, after Boromir dies, and starts to stand up, there only  appears to be two arrows in his body at that time.