This mod will modify all default weapons to have three upgrade slots and exposes the weapon upgrade variable in 'XComMod_WeaponSlots.ini' for easy customisation.

Editing Weapon Upgrade Slots
You can find 'XComMod_WeaponSlots.ini' in the config folder.
Feel free to change any of the values to your preference.

Editing Weapon Upgrade Slots for the SMG Mod
You don't need to download my mod for that as the variable is already exposed.

You are looking for three lines which already exist inside the SMG mod.
You need to open "XComLW_SMGPack.ini" which will be either in:
<SteamApps\common\XCOM 2\XComGame\Mods\LW_SMGPack\Config\> If you are using the nexus version or;
<SteamApps\workshop\content\268500\577409322\Config> If you are using the steam workshop version.

Open "XComLW_SMGPack.ini" and change the following lines, once you find them.

SMG_CONVENTIONAL_UPGRADESLOTS = 2
SMG_MAGNETIC_UPGRADESLOTS = 3
SMG_BEAM_UPGRADESLOTS = 3

[NOTE]
While I haven't noticed any issues in my tests, this mod does give red screen errors when analysed via debug mode, so please create an 'unmodded' save of your game before installing this.

To avoid UI errors I wouldn't recommend setting the number of upgrade slots over 3.

---- [XCOM 2 WotC Expansion] ----

Manually Installing the mod
Copy the contents of the mod package to the directory <\Steam\steamapps\common\XCOM 2\XCom2-WarOfTheChosen\XComGame\Mods\>
* If the Mods folder doesn't exist, please create it.

Manually Uninstalling the mod
Remove the folder <\Steam\steamapps\common\XCOM 2\XCom2-WarOfTheChosen\XComGame\Mods\Extra_WeaponSlot_WotC\>

---- [Base XCOM 2] ----

Manually Installing the mod [Base XCOM 2]
Copy the contents of the mod package to the directory <\Steam\steamapps\common\XCOM 2\XComGame\Mods\>
* If the Mods folder doesn't exist, please create it.

Manually Uninstalling the mod
Remove the folder <\Steam\steamapps\common\XCOM 2\XComGame\Mods\Extra_WeaponSlot\>