The battalion has a war diary covering 1945 at the National Archives - details below.
WO 171/5246 3 Monmouthshire Regt. Jan-Dec 1945
It's unlikely that they will mention him by name but you will have a very good idea on what the unit was doing on the day of his death assuming he didn't die of wounds, there may be clues to this in the diary.
Drop me a message if you'd like a copy.
Hi, new to this forum and have found your message. My father served with 3 Mons from 1938 to 46 and after years of opportunity to find out about his service, I no longer have the pleasure of speaking to him. I love a copy of the diaries if possible. thanks, Mike.