<p>Many schools will read all the recommendations you will send, so it will be fine to have 3, but I am sure that you can tell one of the other teachers you’ve asked that you decided to ask someone else, since he is more familiar with your work, and knows you better. I doubt that they will be offended.
When choosing the teacher to write your rec, the most important is that this teacher knows you well, and LOVES you. This is especially true in your case, when there is a less-then-stellar track record to overcome. The best recommendations are very personal, and include specific examples and anecdotes, and talk about you as a person. It is not important if the teacher is the head of the department. It is important that he/she has something special to say about you.</p>