I'll expand on Cynic's creatine comment.
creatine is not a 'fast acting' supplement. creatine naturally exists in the muscles...supplementing just boosts those natural levels..but it takes time. that's why there's the option of a loading phase.
however you can also simply take a teaspoon of creatine every day, 7 days a week, rain or shine....to achieve saturation in the muscles. this takes about 3 weeks of daily dosing.
But if you only took it 3 days a week, that means you'll be losing saturation 4 days a week. you'll never get saturated, and never get the benefits of creatine supplementation.
I would also say its unwise to ever ditch consistency in your diet. the body works best on trends in the diet. if you skip your protein shakes, and don't replace those liquid calories with solid calories, you end up with fewer calories eaten that day. Your body won't see the 'general trend' in calories, and will be less responsive to your training.
Now, you haven't really said what your goal is....just that you're weight training.
If you don't care about putting on muscle, or shedding bodyfat, then you can be a little more 'lazy' about your diet and consistency with it.
but if you have specific body composition goals in mind...then you really need to stick to your game plan 99% of the time to see the best results.
This is important, because when you see results, it keeps you motivated and moving towards your goal, as well as prompting you to set new goals.
