Never download steamapi64.dll from a random "DLL download" website. Those files often contain malware. You should only get this file from an official Steam installation or a trusted friend who has a working copy of Black Ops 3.
This error is not a sign that your computer is broken, nor does it mean your Black Ops 3 installation is corrupted beyond repair. In fact, this is one of the most common—and solvable—issues for PC gamers using Steam. This guide provides to permanently fix the steamapi64.dll missing error in Black Ops 3 . Never download steamapi64