~/.config/karabiner/assets/complex_modifications/karabiner-thinkpad.json

karabiner-thinkpad.json

{
  "title": "Thinkpad keyboard",
  "rules": [
    {
      "description": "Button3 => Cmd + Button3",
      "manipulators": [
        { "type": "basic",
          "from": {
            "pointing_button": "button3"
          }
        },
        { "type": "basic",
          "from": {
            "pointing_button": "button3",
            "modifiers": { "mandatory": [ "command" ] }
          },
          "to": [
            { "pointing_button": "button3" }
          ]
        }
      ]
    }
  ]
}