You do triplanar on landscapes the same way you do it on meshes.
You need to create triplanar UVs (multiple tutorials on this) and then adjust the sampling/number-of-texture samples you use.
So where you calculate your UVs, add your switch between the UVs and the texture-samplers, and for the 2nd path, plug in your triplanar UVs.
For the texture-samplers, do the same thing. If you are using a single-sample you would keep it the same, and if you are using 3-samples, then add another path like above.
Past that, mathwise, is where you will still do all your specular, etc, etc.