I'm going to be applying for postdocs in the immediate future in either physics or pure mathematics (preferably the latter, though my focus won't change). Assume for the sake of argument the following two letters are strong: (1) my PhD advisor (a physicist by training) and (2) a lesser-known mathematician whom I've been collaborating with recently but haven't produced anything yet. Now, suppose I need a third letter.
My interests have often brought me into collaboration with people in a different disciplines. Do I choose the third letter based upon how well the third person knows me or based upon the discipline of the job I'm applying for?