Using the proper form is a very important factor of muscle growth and strength gain. However, probably 70-80% of the people training in the gym don't always use proper form! I cannot stress enough how important using proper form is for each and every one of your exercises. Yes, it is a pain having to remind yourself every exercise and focusing on form when you would rather focuse on the weight. But believe me, if you start using proper form you will see some fantastic gains. So here are the common form mistakes when performing "The Big Three".
Bench Press =
First of all, using your legs and entire body to push the weight up does not help you develop your chest. It might allow you to lift more weight so that you can brag to your friends, but what good is that when you don't see any real gains in your chest size? Not very good at all.
Remind yourself to keep your buttocks on the bench at all times and your heels glued to the floor. Doin so will allow you to target your pecs more and it means you won't be doing any cheat reps. The width of your grip on the bar is also very important when targeting the chest. Don't have a close grip or a wide grip keep your hands in between these two areas. You need to find the spot that allows you to go through the full motion while placing maximum stress on the chest.
As for arching your back, when it comes to lifting a large amount of weight a SMALL arch is fine. When your not lifting a massive amount you shouldn't have any arch in your back. Often people over exaggerate it so that they can cheat and use their lower body to lift the weight also. Remember, there is no point in injuring yourself just to look cool infront of your friends by attempting to cheat lift alot more than you can actually lift properly.
Also you should ALWAYS have a spotter when using a heavy weight load. You cannot do half as much reps safely as you can with a spotter.
The Squat =
Squatting is very underestimated as a means of growing your physique as a whole. Many people just consider squats as an exercise which grows just your legs. But this is extremely wrong. The reason you won't see so many people at the gym doing squats is because they are not easy! Going "ass to the grass", so to speak, is not easy.
Anyways, onto the common mistakes of form when performing squats.
Do not believe people when they say don't do full squats "because it hurts your knees". Nonesense! It develops them and makes them stronger. Partial squats are actually the exercises which cause injuries not full squats! The best thing you can do is squat with a small amount of weight on the bar so that you can perfect your technique and form first. If you squat with good form you will never need to worry about injuries or hurting your knees.
Curving your back can happen if your trying to lift a lot of weight, to fix this form mistake you should have a wider stance than normal. Wider than shoulder width is fine. This allows your full body to lift the weight, giving you more overall power. Also, your chest should always be facing upwards, you should be pushing with your heels, and keep your head focused on one point infront of you.
The Deadlift =
Another exercise that can transform your body fairly quickly when using proper form and progressing fairly regularly. As with the squatting, doing deadlifts is dangerous, but as your technique gets better it get less dangerous and the chance of injury lowers. Therefore, you should start off lifting light to work on your form and get heavier whilst maintaining your perfect technique.
Your goal when deadlifting should be to keep a straight back whilst being able to lift a heavy weight. You should be pulling ur shoulders back, lifting your chest up, and looking straight forward.
Thats all you need to know when it comes to performing the big three with perfect form, now go to the gym and practise, practise, practise!

Author's Bio: 

Author of fitness and bodybuilding articles with 8 years of experience in health and fitness. Also has experience in household items such as wood storage shelves, Nursery Shelf and Hanging Glass Shelves items.