There are a few mistakes beginners tend to make when doing the squat, bench and deadlift. This doesn't mean everyone has these issues, but just they seem to be the most common based on my experiences. This is obviously a bit biased and may be specific to the population I work with. Also in this context, I define beginners by people who have received basic instruction on how to execute the lifts, but have little experience and practice actually doing them.
Squat is usually a tough movement for beginners. People are not used to the movement pattern and being in a seated position most of their lives certainly doesn't help. The most common mistake is usually leaning forward on the way up.
This may indicate a quad weakness, however, for beginners it's usually not the case, it's simply a matter of technique. It usually occurs because people aren't tight enough on the bottom, and easily lose posture when coming up.
This often happens from rushing the descent. There's no need to rush, take your time, brace your core as hard as possible and sink in. After coming up, maintain that tightness. It helps to consciously think about not letting your chest collapse. A common cue is "chest up", although I've found "driving the bar back" to be more successful. Pause squats, tempo squats and and pin squats may be a good tool for reinforcing maintaining tightness and proper position.
Make sure that all your reps have good form. If your form starts to break down (which is somewhat normal as you get fatigued), lower the weight until your technique is correct. You have to practice good technique to really dial in the motor pattern. Grinding good-morning style reps will get you nowhere.
Showing the chest collapsing. Image from Starting Strength.
In the bench, I was unsure if I should put tightness at the bottom or flaring/tucking the elbows. Since I'd say it's about 50/50, I chose the latter so the entire article isn't about tightness. When it comes to flaring their elbows, it's usually either too much (usually when coming from a bodybuilding background) or too little (usually by the advice of geared powerlifting articles or videos). On top of the right angle, it should have a right timing as well. Your elbows should tuck slightly when the bar is coming down, however, they should flair more once you start pressing off your chest. This makes the movement both safe and efficient.
Most novices just don't tuck on the way down, or if they do, they try to maintain that on the concentric as well. The amount of tucking/flaring is individual and depends on many factors, but as a general guideline when the bar is descending aim for about a 40º angle. In the concentric part, you can let the elbows flair slightly to about 60º.
Remember that this isn’t set in stone, there’s no magic in this specific numbers, feel free to experiment.
For the deadlift, it's by far the setup. About 9 out of 10 times, when watching someone deadlifting I can tell if their technique is going to be good or not based on their setup alone. The bar hasn't even left the floor, yet most of the times I can visualize how the rep will go based on their position and tightness.
Take your time to setup. Watch the best deadlifters in the world because the vast majority are technicians. Their setup is incredibly meticulous and they're about as tight as humanly possible. Make sure you're in the correct position in relation to the bar and you're tight/braced before starting to pull. Pull the slack out of the bar and don't yank it. This is super important and almost every single beginner does this.
This takes time to learn and won't fix itself overnight. But as long as you focus on it, it will improve over time. Doing more singles than usual (doesn't need to be heavy) before or after your normal rep work can be useful so you can better practice your setup each time. Make sure to record yourself so you can see your mistakes and what can be improved.
Beginners are prone to make simple mistakes, but everyone goes through it. You just need to be aware of it and try to improve. It will take time, but you will eventually get there with hard work and consistency.