This is a very expensive attack, it would require 3000 routers to be set up to get a 1/3 chance of catching timing data. Some believe this is how they got Silk road. The tiny number of nodes compared to users makes it a lot cheaper than it could be if there was any incentive to run relays.

l0k18

bitcoin

How Did the FBI Get a Tor User's IP Address?

satdeck

The only thing that springs to mind is a Sybil attack, which is a major vulnerability of Tor. Basically, if your message is encrypted by 3 separate computers (relays), then feds could simply pose as independent relays themselves. By relaying your message between each other, they could effectively deanonymize you.