It is a well known communication practice to divide content you want to be memorable into series of threes. I don't know why but it works whether providing a soundbite for broadcast, as part of a speech or presentation and also as a way to package up a policy.
For example, 'Government of the people, by the people and for the people' or what about 'Education, education, education' or 'I came, I saw, I conquered' and earlier in the Republican presidential race Herman Cain introduced a '9-9-9' tax simplification plan. There are many more examples throughout history and every good speech will include lists of three.
So Rick Perry was right to list the three federal agencies he would target to cut the budget deficit. Three is a great number for communicating a policy as it makes that policy memorable...unless the candidate himself forgets the policy in the first place: