What does it mean when the furnace light indicator flashes 3 times, meaning pressure switch is closed. What do I do? Nordyne furnace M1M series.

Your pressure switch is stuck closed or you have a restriction in the drain line keeping it from reopening(froze drain line). If it's a stuck pressure switch you can blow and suck softly on the rubber line going to the pressure switch to help free it up. It doesn't mean the switch is bad or going bad it just happens some times.

Pressure switch closed with inducer off just like T-top said.

Here is a service manual for your unit.

http://www.nordyne.com/Literature/690a.pdf