At the final rotating pair of spiked balls, just before the "X", are two small pillars with a wall that you can Spin Dash through. Once through, return to a thing that sets you spinning, then go to Knuckles to break through the pillars beside him.
When you reach the third part, just before the X Mark, go off the nearby cliff. Then, go down where the Bumblebee Badnik is located. Destroy that little object with the grass under it. Then, you can get to Tails. On the first part, go to where the Blue Shield is located, next to the first exit. Collect 50 Power Rings, then Spin Dash into that wall that looks pushed back slightly. If you won all the Special Stages so far, with six Emeralds you will get the Last Chaos Emerald.
Press M during game play to enable the "Escape" menu. Then, press [Esc] to blank the screen, with a menu at the top with various options. Press the following keys to activate the corresponding cheat function.
CHEAT FUNCTION - KEY Help - F1 Toggle the map and stats - F3, ENTER, or S Save screenshot in .LBM format in game directory - F9
Also, when the game starts the first time, two files will be written to the game directory: ""report.txt" and "SONIC3Dsonic3d.ini". Both will be empty. To start in any area (1 though 3) of any zone (1 through 7), copy the "Z<1-7>A<1-3>.SAV" file of that zone and area to the file "PCSONIC.SAV". Select "Continue" from the main menu or load that file from the "Escape" menu to start at that area and zone.
Collect fifty rings, enter the special stage, and get the emerald. Lose your current life, gather another set of fifty rings, and re-enter the special stage. Give the rings to the same character used the first time the special stage was entered. The same stage where the emerald was awarded will re-appear. Complete the stage for an extra life. Collect 100 rings for another extra life. Lost your current life and repeat this procedure.
In Green Grove Zone Act 1, collect all the rings. Then, when you move onto Act 2, collect 200 Rings and go through the hidden passageway before the third part of the stage. Collect the extra life and complete part three. After that, when you go to Rusty Ruins, go through the little hole. SpinDash, and get an extra life. You should have your maximum of 10 lives.