They may feel like why do you want to have it in the first place, is it cause they no longer do it for you or is it just to spice up in the bedroom, and then there is the if i do it this time then he/she will want to do it often... So if you really want to do it without sacrificing your relationship then speak with them, let them know its just sex not looking for a new fuck buddy out side of the relationship.
Then it comes to the type of threesome, most straight woman would prefer mfm threesome, though most straight males want fmf threesome, though bi way woman tend to lean more towards other woman in their threesomes.
If after speaking of it and going over it, if they still do not want to, then you have two choices, look else where for a threesome and risk your relationship, or think of another way to spice up your bedroom without bringing another person into it.