Nilador
Vikings would win. They'd watch the pirate and ninja fight and would just kill the loser and take the winner as a slave.
Well you assumption is that the pirate and ninjas would fight head to head, but this is wrong. Historically pirates and vikings are both raiders and therefore the more warrior type peoples. Viking however didn't have guns and pirates did, also pirates had bigger and better ships because of technology advancements.
Therefore historically and technologically speaking the pirate would raid the viking (because they could travel further and faster) and pirates would then devastate the vikings. And I say this with part of my family lineage coming from Finland.
Next ninjas hid in the shadows waiting for there opponents and did not just charge out. When the pirates walked by they may be able to kill a few, but after that it's over. Ninjas give away their hiding place and pirates shoot them. Because of technology the pirates would win hands down.