You want a derailleur for a road triple crankset, and specifically one for road shifters. (Apparently the cable pull is not standardized for front triples.) In your case, your bar end shifters should be road shifters. So a Sora or Tiagra triple front derailleur should work. They're both optimized for 50T big rings, but that's close enough to count - 2 teeth isn't going to make a huge difference in the circumference. The only question will be the 20 tooth total capacity rating. That's a vague number, which can usually be exceeded by at least a few teeth.