It has nothing to do with Christianity or the Celtic Trinity. If anything it's likely a Japanese mon; it could, logically, be the kamon of the Iori family.
You'll see one on there that's very similar to the symbol on Junpei's hat, just pointy. There are thousands of mon and kamon out there, so it's likely there's one that actually matches the one on Junpei's hat.
Or it could just represent a fictional baseball team.