Why do I have a large PVC pipe venting warm air outside intermittently?
Do you have a high efficiency furnace? If so this is the exaust and air intake for your furnace.  Unlike older furnaces, HE furnaces do not exaust up through your chimney and instead use PVC or ABS pipes.

Not sure about the third pipe though - do you have a heat exchanger?

Best way to know for sure what they are is to try and trace them back to the source.