My wall outlet is a 4 prong I have a pigtail to change my Sears dryer to a 3 prong but not sure how to properly wire it the 4 plug is red, white, black and green the replacent not color coded

The middle wire on the 3-wire pigtail is the neutral and goes to the middle terminal on the dryer. The other two wires go to either of the outer terminals (where the red and black wires were with the 4-wire cord). There should also be a bonding strap that needs connected to the middle (neutral) terminal to bond it to the frame. The strap is probably there but wasn't connected with the 4-wire cord. If you don't have the bonding strap, you can get one at Sears.

My wall outlet is a 3 prong and I want to convert it to a 4 prong but I only have three wires?