If you mean THAT kind of three-way, hell no. If it was a date with me, L and Light, maybe. But only if Light remains silent and does nothing throughout the entire thing. GO BACK TO THE SHAME CORNER, LIGHT! D:<
Me: Do ya mean that you and i will be on a date while Light is here?
Me: Hm....I don't trust him!! *glares at Light*
Light: *glares back*
Me: I'll only allow this if you promise to make out with Light. (lol yaoi fan)
I'd go, if only in the name of curiosity (because, what does a genius think a "good date" is?), and when he brought Light, I'd think something was wrong. You know, "gotta work, sorry, date's off" or something. Upon the prompt to the three-way, I would, and yes I'm ABSOLUTELY CERTAIN of this, die of laughter.
I would burst with a snort first, because you can't hear something that absurd without it catching you TOTALLY off guard and causing an eruption too big from the get-go to hold back, and it would become instant hysterical laughter. I may actually bend over and roll on the ground laughing. Before dying.
Then I'd get up, still laughing, but more manageable, pat him on the back, and tell him what a great sense of humor he has. Then chuck a gentle fist into Light's shoulder for being such a good sport as to play along.
(and if I found out they were serious, I don't know if I'd run screaming, or suggest a change: "let me draw you two naked." ;D)