Threesomes

I’m a bisexual female and my partner is interested in a threesome with another girl. I am interested in having sex with a girl in my lifetime, but in also worried he will be more interested in the other girl than me or it will affect our sex life after. I don’t mind the idea of letting him watch but I am concerned about sharing.

I told him I’d be open to thinking about it. But I just am stressed. I dont want to let him down but I also am worried I will be miserable and anxious the whole time.

Advice?