G50 S3000, You are "telling" the machine is the highest speed you can reach.
Because G96 is a Constant Surface Speed and according to Diameter being cut
As much as X axis is travelling to center of work piece, automatically spindle speed is increasing until it reaches maximum you have stated, this case 3000 RPM.
X will move to whatever coordinate you writebut need an F.xxxx depends if you are working metric or Inch it will be advance per revolution.
Ie. X-.050 F.005; Normally face cut, X will travel .050" past center at .005 thousands per revoulution.
G96 you are commanding Vc Velocity of cut (Tangencial constant speed)
G97 you are commanding RPM (Constant speed)