First we need to install Plutonium for Call of Duty: Modern Warfare 3
Go to https://plutonium.pw/ make an account on this website as you will need this to install the .exe for plutonium.
When you have made an account, then head back to the main page there is a download button that will bring you to the download page.
Download plutonium.exe, you can save the launcher anywhere you want, somewhere convenient such as your desktop or the game folder.
Open the launcher, if smart screen comes up click More info, then Run anyway.
The launcher will now start the process of installing the client files.

Enter your forum credentials to login, the same account you just made.
Then we will need to setup the game, you can use Plutonium to play World at war, Black ops 1, Black ops 2 and Modern warfare 3.
Navigate to Call of Duty: Modern Warfare 3, click on SETUP and then select your Call of Duty: Modern Warfare 3 game folder.
The folder you need to select is most likely called Call of Duty: Modern Warfare 3 in your steam install folder.
You can easily find it by going to the game in your library, and clicking on the gear on the left, then go to installed files, show file location.
Now Plutonium is fully installed.

Next will be the offline launcher.
Go to https://github.com/JugAndDoubleTap/LanLauncher and download the latest version of the Lanlauncher
Extract the LanLauncher file onto your desktop, Personally I have a folder with both the Lanlauncher and plutonium launcher.
And launch the Lanlauncher it will bring up a menu that will ask you where you have your game installed.
This launcher will also work for Plutonium World at war, Black ops 1, Black ops 2 and Modern warfare 3. (Which I will make guide for as well on how to play offline with bots and have progression)
Just browse to the install folder of Call of Duty: Modern Warfare 3 and save, then fill in your Username that you want to use in game and make sure you select the option for multiplayer.
Now you can click ''Launch IW5''
You can use this launcher to start up Call of Duty: Modern Warfare 3 offline when you are disconnected from the internet.
Even if you are connected to the internet, it will skip Plutonium searching and installing of updates.

Now for the full Bot experience.
Go to this website and download the [MW3] Bot Warfare v2.3.0 LATEST https://www.moddb.com/mods/bot-warfare/downloads
Make sure to have a program that can unpack/extract .zip files (WinRAR, 7zip, etc.)
In the unpacked file is a install batch file, run it and it will install the mod for you.

Now run a private match in game on whatever map you want with whatever settings you want, once in a match.
You can open the menu by pressing the Action Slot 1 key (default 'N', nightvision key).
You can navigate the options by pressing your movement keys (default WASD), and you can select options by pressing your jump key (default SPACE).
Pressing the menu button again closes menus.

Now in the match it will use random names for the bot players, the same as in private matches normally.
You can change the names by going to C:\Users\User\AppData\Local\Plutonium\storage\IW5 and adding a textfile name ''Bots''
Here you can type in names you want to see in the match, each name can be a maximum of 15 letters/numbers/symbols before being cutoff, also these names wont have the trademark bot clan tag in game.

For MW3 you need LanLauncher version 1.1.8, version 2.0.0 doesn't work for MW3 specifically.

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3498442654