Here are the instructions:
1. You will need 4 things: An objective item (flag, ball, bomb, etc), two weapons of your choice (NOTE: one of these two weapons has to be dual-wieldable), and a SMG.

2. Place the SMG on the objective item either in-game like in the video or in forge.

3. Start the game with the 2 weapons that you chose. Switch to the dual-wieldable weapon (mauler, plasma pistol, plasma rifle, etc).

4. Go to the objective item (which has the SMG on it), and press LB and RB at the same time.

5. Drop the objective item, and you are now holding three weapons.

By the way, you cannot switch out the weapons otherwise you’re back to your original 2 weapons setup.