Have. Would again.
Clearly our experiences differ. 
In response to the OP, though, my response would likely be “No” because of the high probability of it being some sort of set-up.
What’s with you guys who worry about getting robbed or chopped up? A single woman can do that too, you know. Live a little.
So, yes, I’d go for it.
May be really good fun.
Worst case scenario, it isn’t. But thousands of other things aren’t either.
But is there a proviso that I die if I don’t enjoy it?
Wow, I’m surprised by a lot of these responses. Not the answers, but the reasoning behind them. Is it common for men to get robbed/murdered in this way?
I voted no. I’ve been in exactly one threesome in my life. Never again.
Might be TMI, but what was so bad about it ?
It depends. Did we share a few drinks and laughs before the offer was made or did they just walk up to me and say, “We saw you from across the bar and decided we want you to fuck us both”?
This
At my age and stage of physical conditioning (or lack thereof) I’d be more than suspicious…
No, but we probably have some ex’s who fantasize about it. (The murder part, not the three way.)
No, but it’s not common for two strange women to walk up to me in a bar and ask me for a three-way. I’d figure there was some sort of weird motivation behind it.
Actually I get asked this exactly at my local poolhall pretty often. Turns out they’re for real, but end up stealing my wallet. It would be hard to say I’m getting sick of it, but this is getting out of hand. I got my wallet stolen Last Friday, Sunday, Monday, Wednesday, and agin last night. * D&R*
So what the hell happened on Tuesday? Couldn’t find a spare wallet?
To paraphrase Dr. McCoy: “Dammit, Jim, I’m a stud, not a circus contortionist!”. 
Actually, it was fun because it was something new and unusual for me. However, it made me realize that I am a one-woman-at-a-time guy.
If I weren’t married, protection is involved, etc… what the hell? You only live once.
Or in this case, twice. 
Reading the thread title, the image that came to mind was George Costanza, handcuffed to the bed while the the woman who conned him goes off with his wallet.
How does the German Shepherd feel about it?
I once turned down a foursome, three women and me. I didn’t find any of them very attractive and was afraid of an unintended pregnancy, figuring that would triple my chances.
I was firing up my motorcycle after a band finished up at a large outdoor festival.
A stunningly beautiful young lady walked up and asked me if I could give her a ride to her car which was a couple blocks away. She was wearing high heels and said her feet were killing her. So she hops on and I take off, folowing her directions which were leading to a rather dark and deserted alley away from the festival location. I got real nervous figuring there would be a couple guys there waiting to relieve me of my bike and wallet but…nobody there.
I did get an immediate “OK, I’m done with you, fuck off” vibe the second she got off.
Tuesday’s Bingo night!