Finally got MechWarrior 3 working on Windows 7 (64bit Pro). Here are my settings.
Compatability: Windows XP SP3
Game: Self-Shading=Off, everything else On or High, 8MB textures, 1024x768. Theme/music set to Off (This stops the game from accessing the CD for the Music.)
Video card settings.
Vertical Sync. Force ON. (eliminates bouncing APC's, broken physics) Ambient Occlusion=Off Anisotropic Filtering=Off AA Gamma Correction=ON AA Mode=Enhance the Application Setting AA Setting= 8x AA Transparency 8x Supersample Max pre-rendered frames=4 Negative LOD bias= Allow Texture Filtering=high quality Threaded Optimization=Auto Triple Buffering=On
Note: There is no AA in MW3. Setting AA to "Enhance the App's Setting" seemed to eliminate an occasional CTD.
This is on a Nvidia 550 TI 1GB DDR5.
Only bugs are rare APC jumping and an occasional crash if I watch the mission intro video, but quitting to the main game page after viewing the intro and then going back to launch mission eliminates this.
Pirates Moon also works with the above settings. Only bug continues to be an original release bug where Pirates Moon doesn't recognize the Throttle on USB Joysticks.
I hope this works for you. I am guessing these settings would work for Vista and XP also, but haven't tested it yet.