Can I master drawing in 3 months?
First, I will not say it is impossible, but I will say it is highly unlikely that you will be able to "master" the art, skill, and (dare I say) 'science' of drawing in 12 weeks.
Let me explain: it takes a person hours of practice to become a master of anything. Assuming you have little or no prior drawing experience, mastering drawing in three months would be like mastering a sport like soccer or basketball in three months.
According to a widely referenced 1993 paper written by Anders Ericsson, a Professor at the University of Colorado, it takes a person 10,000 hours to become an 'expert' or 'master' at virtually anything. I've heard there is a sort of mantra around Pixar that 'everyone has 10,000 bad drawings inside them' and that to become a decent artist, you first have to get them out. I appreciate this philosophy. Of course, you have to make an honest attempt at a drawing for it to count. This is not to discourage you, rather to possibly motivate you.
Of course, you could accelerate this process in many ways such as:
(1) Getting a mentor. I do not believe in 'talent'. One trait common to practically every 'child prodigy' from Mozart to Picasso is not innate mastery of a skill, rather a mentor. In fact, both of them were blessed to have talented parents. Picasso's father painted birds and Mozart’s father had created several musical compositions himself. This is further described in Geoff Colvin's book, Talent is Overrated. The point is, talent is a myth. Picasso and Mozart's parents may have passed their skills to them but not genetically.
(2) Getting an education. Whether through books (I strongly suggest Drawing on the Right Side of the Brain for starters), online programs, or traditional classes (check out your community college), education, mixed with passion and drive, is possibly your best catalyst. Just an additional note: if you can draw the human figure in all its complexity, you can draw anything. Try to find a life/ figure drawing class.
(3) Drawing everyday (and everywhere). If you want it badly enough, you can find some time to do this, even if it is only for 15 minutes. Draw things you see in your everyday life, from observation. Draw less from pictures unless you have to. Draw things you've never drawn before. It doesn't have to be perfect and it won't be. Just keep a sketchbook with you at all times and draw whenever you have time and I can guarantee you will see results.
One thing I have noticed is that artists often try to make drawing sound like it is a selective ability. THIS IS A LIE! Don't let anyone convince you that you are not one of the 'chosen ones' to be an artist. It's not a selective club. You just have to practice.