I just did my first partigyle over the weekend, which I did a ton of research for. Everything I read suggested NOT doing "extra" sparges. Collect your first runnings, that's your big beer. Sparge and collect your second runnings, that's your small beer.
If you're splitting the recipe in thirds where the strong beer is 1/3 of the total volume and the weak beer is 2/3 of the total volume, you can expect the strong beer to get about 1/2 of the sugars and the weak beer will get 1/2 of the sugars. So, since the first 1/3 is a smaller volume, it'll be more concentrated and stronger.
You can break it into three runnings, but it always has to tie back to your original "master" recipe. So if your bag of grains is good for 5g of 1.050 wort, you can do 1.67g of 1.075 wort and 3.33g of 1.0375 wort or 2.5g of 1.0666 wort and 2.5g of 1.0333 wort.
Read the Randy Mosher partigyle article and check out the Braukaiser partigyle simulator. Those two items REALLY helped me wrap my head around the whole thing.