I have ran into 3 dragons before but never fought 3 at the same time.
When i was a low level around 12-13 i ran into 2 giants just waling on a main path "No giants camps were in map view" well i got to close and they pulled out their clubs and gave me the run down..well while running for my characters life i ran into 2 dragons...lucky for me the giants stopped trying to kill me and went after the dragons.. in the middle of their fight a 3rd dragon fly’s over..it didn't attack it just looped around the sky for bit then flew off...the giants ended up crushing the dragons and with the giants having more then 80% of their health gone they easy to take down.
anyways i don't think you can fight 3 dragons..you can run into 3 but i think 1 will always end up running away.
I'm ps3 by the way...who knows if you can on PC or Xbox.