If updating the Steam client doesn't work, you might need to reinstall Steam.
This is the that solves 90% of cases. Steam has a built-in repair function that checks for missing DLLs. If updating the Steam client doesn't work, you
Reboot after completion, then verify game files again. If updating the Steam client doesn't work, you
Add the Black Ops 3 folder as an in your antivirus settings to prevent it from happening again. Update Microsoft Visual C++ Redistributables : If updating the Steam client doesn't work, you
By methodically going through these steps, you should be able to resolve the steamapi64.dll missing error in Black Ops 3.