If I understand correctly, your are replacing a GFCI receptacle with another GFCI receptacle
And your existing GFCI has 3 black and 3 white wires
And there is not a switch involved in the changeover.
I downloaded the manual from Lutron
http://classic.lutron.com/CMS400/techinfopage.aspx?id=22163Every wiring diagram in manual shows:
Black wires connect to brass-colored screws
White wires connect to silver-colored screws
The brass and silver colored screws are typical of all residential receptacles. The color-code tells the electrician which terminal gets the black and white wires.
Are you having a problem putting 3 wires onto 2 screws?
It is a common problem.
You can buy a short piece of white and black wire at hardware store
Lets work on the black wires as example:
Strip back insulation on short piece of black wire
Hold together two of your black wires and start twisting them together
Now add the short wire to the two black wires you started twisting
Now twist together the two blacks and short black
So three wires are twisted together and you have a short lead that can connect to screw on GFCI
Put a wire nut over the twisted wire
Now you're ready to connect other end of short black wire to brass-colored screw
This reduces the number of wires that connect to screws
http://waterheatertimer.org/How-to-twist-electric-wire.htmlhttp://waterheatertimer.org/How-to-select-right-wire-nut.html