Yes take supplements on your off days. Same diet/supplements on off days as you have on on days. The above recommendation to work all your body 3 times a week might work for you...but I don't like it personally, I prefer splitting.
If you want to gain anything, a little bit of muscle, a lot of muscle, whatever, you still need to train extremely hard and eat a lot.
Start by telling us your body weight, your body fat %, your height, and your age.
For example I'm 81kg low bodyfat 22, I need 200 grams protein a day, 400 grams carbs, 200 grams fat, and about 4000-4500 calories to put on muscle. Which means I need to have about 7 meals a day and about two protein shakes.
What supplements are you taking?
Basics you need are Protein, Glutamine, and Creatine.
Good luck anyway.