cola, first make absolutely 100% certain that this is ok with your boyfriend. If it's not and you go ahead with this, then you could damage everything. That discussion should include talking about potentially having sex with him, and falling in love with him (those are two different things, and different people react differently to each).
Do your boyfriend and your best friend get along well?
If the three of you hang out, then one way to do it is to bring up in casual conversation this new thing that you have discovered called polyamory, and has he ever heard of it? If not, then you can explain it - do it as an academic exercise at first - something neat you've heard about and what does he think.
If you get a response like "Ewwwwww!" or "Well, fine for others, but I couldn't", then you know not to push it any further.
If the reaction seems positive I would then change the subject - you're not in a rush, are you? Afterwards, talk to your boyfriend about how it went and how he felt having the conversation.
Then at some point, bring up to your best friend that you are attracted to him. The rest will depend VERY much on how he reacts to that.
Maybe others have other ideas...
"Listen, or your tongue will make you deaf." - Native American Proverb