As you already have a working dipole (which you have in this case), the primary thing to consider is the element lengths and spacings. If you don't have one of those routers with massive (and empty!) plastic antennas, there's probably a good chance that you can use the authors design parameters.

Otherwise, there's plenty of 3-element Yagi-Uda calculators online where you can stuff in various amounts of parameters and get the unknowns—the author used fancy antenna simulation software, but you don't need that.

A few links:

http://www.rfwireless-world.com/calculators/3-element-Yagi-A...

http://www.changpuak.ch/electronics/yagi_uda_antenna_DL6WU.p...