First of all you need Atari800WinPlus3 emulator ( click here to download).
Then you need River Raid ROM Image ( click here to download).
STEP 1. Run Atari800WinPlus3 emulator and load ROM image using File/Autoboot image.
STEP 2. Run River Raid by pressing start button/key ( F4 usually ).
STEP 3. Go to Misc/Cheat otions... menu.
STEP 4. Enter memo value 3 and press Memo button in 'Cheat option' dialog box.
STEP 5. Enter search value 3 and press Search button in 'Cheat option' dialog box.
STEP 6. Check address $006A, Check Lock in, Set Lock in value 3. Press OK button.
STEP 7. Now you have 3 lives permanently. Enjoy the River Raid - the best scrolling shooter ever :).
CONTROLS by default :
Fire - Left Ctrl
Left/Right/etc. - num pad keys