Don't take this the wrong way, but those lifts are absolutely pathetic for someone who has been lifting consistently for 3 at 20 years of age. I am not doubting your resolve, dedication, or even training intensity, as I know nothing about you in those areas, but what I am saying is that (medical issues aside) there is something MAJORLY wrong with your program, as even someone with the worst genetics should be squatting and bench pressing much more than that after 3 years of consistent training.
Like several others have said, it is impossible to tell you why you stopped gaining strength without knowing a whole hell of a lot more about you. We know nothing about your metabolism, diet, training program, etc. Whenever I see someone in your situation, the extreme lack of progress is almost always due to ignorance regarding the basic principles that govern muscle growth--namely diet and/or training. While I have seen many people with poor training programs, horribly inadequate diets are even more common.
If I had to guess, it sounds like you need significant coaching in multiple areas--and that you should invest in a coach, even if just too instruct you on the basic principles mentioned above. There are certain things that must happen in any training program in order to accumulate muscle tissue, and if they aren't, it doesn't matter how meticulous you are in any other area--you will fail, horribly.
While there is much ti learn in the areas of training and diet, here are the most basic principles require for growth.
Diet: You must eat above maintenance--meaning a caloric surplus. Find your caloric baseline (the calorie amount at which you maintain your bodyweight) and add 500 calories to it per day--every day. You can make adjustments from there as needed. Not essential like the above, but still of great importance if you want to gain muscle at an acceptable rate, is to eat enough protein. 1 gram per pound of bodyweight is fine. The rest of your diet should be comprised of carbs and fat in a ratio ideally suited to your metabolism, but you will learn that as you go. For now, eat above maintenance and consume adequate protein--every day. As for carbs & fat, try to consume the majority of them from healthy sources, according to your own understanding. Split your daily calories fairly equally into at least 4 different meals, while also splitting your daily protein intake fairly equally among those meals.
Training: Progressive resistance should be your guiding light. In other words, you want to get stronger within the hypertrophy rep range, which for you, should mostly fall in the 6-10 rep range for upper-body and 8-12 for legs. Of course, many people travel outside of those ranges, but they will suit you well for now. In terms of program set-up, there is no one program that is the best. There are also many different training styles and specific programs that have produced great results for many people. Some good ones are DC Training (dog-crap), Mountain-Dog (John Meadows), and Fortitude Training (Scott Stevenson), but there are only a few. More traditional programs have also worked great for huge numbers of people. All effective programs have one thing in common--they properly balance training with recovery. This is a very broad subject, so unless you understand how to do this (my guess is you probably don't, at least not very well), I would select a "pre-made" program from someone who does.
Sleep: Try to get at least 8 "uninterrupted" hours per night, if possible. Sound sleep (which means completing an optimal number of sleep cycles) is an important factor in optimizing muscle growth. If you have trouble sleeping--fix it.
If you do the things above, you are virtually guaranteed to make both size and strength gains. You are not ready for steroids, but if you want to take a supplement that will give you a noticeable boost, use creatine. Plain ole' creatine monohydrate works great. It helps stimulate growth through multiple direct and indirect mechanisms and it's price is so reasonable ($5-7/month) that everyone who is serious about muscle and strength gains should be using it.
I realize this is all very basic information, but these three things (training, diet, and rest) make up the foundation on which any successful program is built. Without adhering to these basic tenets, you will go nowhere fast.
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