wireing a dryer plug that has 4 wires from l-14-20amp twist lock plug to a 3 wire dryer plug?
Answers:
DO NOT interchange the green (or bare) ground with the white neutral. Even though they are connected together (at a single point, the service), they service different purposes. The white carries current and the ground does not. They should never be connected anywhere other than the service.
I'm not following from your question for sure what you are trying to do. First, a standard household dryer is 30 amps, not 20. And why does it have a twist lock plug on it? Then you say the dryer plug is 3 wires. Sorry, please explain what is 3 wires and what is 4 wires. You do not mention a receptacle anywhere.
Here is standard installation of dryers, which might help. A dryer receptacle installed in a house before the 1996 code was adopted is generally 3 wire. Outlets installed after that are required to be 4 wire. A dryer normally comes without a cord, and the electrician will install a 4-wire or 3-wire cord to the dryer to match the outlet. Inside the dryer are connections for the 2 hots (red and black), the neutral (white), and a ground (green), which is connected to the frame of the dryer. The dryer may ship with a strap between the ground and the neutral, which is correct for a 3 wire cord (there is no green wire in the 3 wire cord). That strap is removed for a 4 wire cord. This is not contrary to my warning above, it is not connecting the ground from an outlet to the neutral, it is connecting the frame of the dryer to the neutral instead of a separate ground. This is less safe than the 4 wire connection (hence the code change in 1996, don't ask me why it didn't change earlier). But it is not downright dangerous, which connecting a neutral to a ground conductor would be (past the service).
If you don't understand what I'm saying, then you should have an electrician do whatever you are trying to do.
Edit, now I notice you asked about transfer switches and generators before. If you have a transfer switch you don't have a 3 wire dryer plug. BACKFEEDING A dryer plug from a generator is EXTREMELY DANGEROUS and you should not even try.
Even with a transfer switch, the ground connections at the generator etc are very complicated, and you should have an electrician do the work for you.
Your last question says you have a transfer switch. If you do, see my answer to that question. If you don't, get an electrician to install one for you.
Other answers:
It sounds to me like you are trying to change a 220 volt dryer plug to a 120 volt dryer plug-it won't work and it could prove to be dangerous.
It sounds to me like you are trying to change a 220 volt dryer plug to a 120 volt dryer plug-it won't work and it could prove to be dangerous.
your green and your white neutral wires go back to the same place at your breaker panel. they are both grounds. Therefore your green will hook up at the plug with the white wire.
the green is probably just a ground, just let it be cover it up though so it dont touch any metal or another wire.