Triple cotyledons (1st seedling leaves) instead of double are the result of a mutation carried in the DNA of the seed. Since I start most of my plants from seeds, instead of buying plants, I've come across this a number of times. I've also had some that grew the double cotyledons & then stopped & no true leaves ever formed. I waited for weeks & the seedlings stayed healthy but wouldn't grow. I gave up & pulled them out.

If you're interested in keeping it, go ahead. There's no harm. But in my experience, seedlings that show strange quirks like that usually don't produce well. Whatever caused it to mutate may affect other features of the plant such as disease resistance, taste, productivity, etc.