It’s easy to confuse friendship with deeper feelings, human connection is weird.

If you’re not romantically into him, you won’t be in a thruple, you’ll simply be sharing your partner with your friend.

Monagomy isn’t for everyone, and arguably polyamory isn’t for many people at all. I’ve seen a few of these work out but it requires very different mindsets and your mindset right now sounds like it’s on the sexual side (considering it exciting) instead of on the relationship/partnership side. Excitement is short lived, exercise caution.

Don’t ask the internet, talk frankly with all parties involved. Be aware that the relationship may already have expanded past monogamy non consensually.