<p>I'm not really sure, but generally 1 superstrong letter is better than 3 lukewarm letters. I am having the same problem too, my primary undergrad advisor got into accident so he can't write a letter for me.
I was hoping that he'd write the strongest letter :(
oh well..</p>
<p>Even so, you are luckier than me. My strongest letter of recommendation was going to be written by a professor who has just died... I am not joking!!!</p>