He's an ex for a reason. Don't do it.

If you are not comfortable with doing this , then don't do it.

I'd be disgusted at that suggestion as his ex tbh...what a douche!

Honestly, its a red flag that he wants a threesome with another girl. It doesnt sound like he loves you

How about just say no, threesomes are troublesome at best and any problem in the relationship might be magnified, they tend to cause hurt feelings, jealousy, no feelings of trust and loss of respect, fear of him liking the other person and leaving you and if he can't deal with you saying no then time to say good bye. provided he truly loved you this would not even be an issue, because he would have never brought it up.