I've been doing some poly-song research lately and here are a couple that came up..
Olivia Newton John, Culture Shock. A rather horrible tune but interesting lyrics.
A really beautiful song - Grace Slick, Triad.
A very sad song about a woman with multiple loves is Joni Mitchell's Cactus Tree.
Fun: Iris deMent and John Prine - Let's invite them over
Chris de Burgh - Much more than this
And a great rock song about threesomes: Joan Jett - The French Song