Sweet potato vine,
Ipomoea batatas, can also have tri-lobed leaves:
http://www.finegardening.com/m...
Not that I'm suggesting that's what this is, I think I too have seen them on morning glories, but just that it will probably be easier to figure out what it is when it has grown some more, and especially flowers.
Edited to add:
There's also
Ipomoea cordatotriloba var.
torreyana
http://www.wildflower.org/plan...
Not sure about
Ipomoea alba, moonflower, which according to articles can have three-lobed leaves but the picture I found didn't look like the same shaped lobes. I read about a couple of other species but no pics.
