open relationship

How can you tell your bf of 3 years you want an open relationship? I know this isn't for everyone so please no bashing! 
I have never cheated before, btw. I just feel like this is so me, and could be so "us" but I don't know how he'll take it, or respond. 
Heeelp please :))
Have any of you ladies done or are currently in this situation?