I would go for Prof. C definitely, and Prof. A. Prof. B as Gecko said will only serve to signal your math skills, which won't be necessary ,I think, if you did well in those analysis courses you mentioned.
Math here is over-emphasized, IMO. However a necessary condition for top admits (at least at a certain level, marginal benefits of math beyond real analysis are "very" concave, I think),
LORS weigh a lot more I think. Because credible
LORs can give adcoms much more info about much more things than math courses. I mean, if you took intro analysis, measure, functional analysis, and the like, and you did great in those courses, well, then you've proven your point. There's no need to waste a letter to say that you're good at math, unless you don't have any other one to send. Use that other letter to say that you are hardworking, creative, talented, or whatever.
Besides, adcoms are more familiar with economists than with mathematicians, so if his/her name can ring a bell with adcoms, then it's a useful letter. More even so if he/she has contacts at some schools you're applying to.