How am I supposed to find 3 recommenders for grad school?
First, I wouldn't make the request via email, although it would be a good idea to request a meeting via email to discuss your plans. We may not be able to put a name to a face from an email, but if you make a point of visiting and reminding them of the course(s) you took with them and how you did, that would go a long way in tweaking their memories. Second, come prepared. Bring copies of your transcripts, your resume (which should include work and volunteer experience), and a clear idea about what your plans are, including with who and what you plan to study. Finally, give the prospective reference time to prepare a good letter (a month is usually the minimum) and a clear deadline for when the letter is to be submitted by. 
(As a side note, I assumed your comment about professors not being able to respond to your email simply meant they hadn't gotten back to you.) Good luck!