I have read that too. Can't remember the site. Where did you hear this?
It's pretty common knowledge among bodybuilders and practically any bodybuilding site or book will mention those three as core "bread and butter" exercises that hit up all your mucles and push your body the hardest. Remember that your biggest muscles are in your legs and back. So you any work out plan that does not dedicate attention to those muscles are severely lacking.
OK if I work out different body parts and I add these to each of my work outs? Is that too much?
Well I split my week up into different body parts (back day, chest day, legs day, arms day etc)
Squats -> Main exercise for legs day
Deadlifts -> Main exercise for back day
Bench press -> Main exercise for chest day
Of course I do other exercises too, but those are the ones I always start off with.
How many reps and sets?
I usually do 4 sets for each exercise and set my weights so my rep range is around 10-12. If I can do 12 I usually up the weights.