How can I find games from grandmasters that end in threefold repetition? When I go to http://www.chessgames.com/, I can search for matches that end in draws. However, I can not figure out how to filter those draws to specifically threefold repetition. How can I find these types of draws while filtering out the rest?
If you have a collection of games in PGN format, you could probably search for threefold repetitions using the Chess Query Language. In particular, you might look in the "Matching positions: the position list" portion of the manual at the provided link; the :sequence keyword would likely be instrumental to a CQL solution.

For an example of using CQL to do this sort of thing, see my answer to an earlier question about seeking out exchange sacrifices in games. The method given there uses CQL to scan games for short sequences of successive positions that have a material difference of an exchange. I imagine you'd be able to do something similar to seek out sequences of positions repeating three times.