Threesome??

Ok ladies, serious question.? Ive been with my fiance about 8 years and earlier in the relationship he mentioned a threesome but of course i rejected because i was young and unexperienced, now that i have experience with a girl i want to surprise him with a threesome for his birthday. But i dont want it to be all about me because its more for us both. Its two things im afraid of. 1. How would I find a girl? 2. What if he rejects it an feels like i did it for me? How embarrassing. I need answers!!! What do I do? Surprise or ask him first.