1) Open up notepad
2) Paste this script:
cd /d D:\
cd Steam\steamapps\common\Call of Duty Black Ops III
start BlackOps3.exe +set fs_game usermaps +devmap 761501926

Where cd is, please navigate to where your black ops 3 install location is. ( https://www.google.com/search?sourceid=chrome-psyapi2&ion=1&espv=2&ie=UTF-8&q=command%20prompt%20cd%20command&oq=command%20prompt%20cd%20command&aqs=chrome.0.0l6.8831j0j7 )

3) Save the file as a bat file
4) Make sure steam is running before running it.
5) Run it.
6) Go to steam you should get a popup in a few seconds.
7) Click OK.
8) Wait a few minutes for it to load.
9) Tada!

In order to run mods:
Where usermaps is replace it with the mod id.
Example:
The free perkaholic mod is: 775553917
start BlackOps3.exe +set fs_game 775553917 +devmap 761501926

I'll have a video up in a hour or so if you're confused.