I've never dated a woman who has been so open minded... Some others may have been so lucky...
A friend at work was once so lucky but he tells me he committed the cardinal sin in threesomes. He started paying much more attention to the third wheel which made the wife anxious and she called time out before he was finished. He always used to say make sure you pay lots of attention to the wife and gf so that she doesn't feel less important than the third wheel.
That advice always made perfect sense to me, but I've never been lucky enough to put it into practice...
When I feel like a threesome, I lay down the cash and have my fun with two hot MILFs for the hour...