Threesomes

So my boyfriend and I of 5 years are looking to spice things up and have a threesome with another female. I have never had a threesome but he has. I know I’m okay with it because it will be fun but every time he talks about it I get nervous and don’t want to take about it. He really likes talking about it especially during sex. What can I do to make myself comfortable with talking about it? HELP!