On the G3 type weapon as soon as you modify lower trigger pack assy. to fit a semi auto receiver, or modify that semi auto receiver to accept a select fire lower trigger pack then you have created an illegal machine gun subject to up to 10yrs in federal prison and up to $250,000 fine. You can have the multi position lower grip assy. all you want as long as you don't have a host firearm it fits on.
What you are considering buying is an unregulated spare part for a machine gun, if you don't have a machine gun to legally use it on, there is not need to have the part.