Replacing an on and off switch with a mechanical timer

I have an outdoor light that is run by a regular wall switch. I want to change this to a mechanical timer switch. The mechanical timer switch has two ports for wires. When I opened the wall switch I found three black wires: one in each of the push in ports and one under a side screw. How do I wire in the mechanical switch that has two ports when I have three black wires attached to the old regular wall switch?