If I use 3 gals of tap water for the wort. Can I use 2 gl of tap water to make up the 5 I need for fermenting? Lancaster home brewing say any good tasting tap water is OK, but does it need to be sterile..like after boiling? I intend to keg my brew.
You will definitely want to boil any water that you intend to add to your fermenter.
If the yeast is already been pitched into your wort, then you should also cool the water back down to your yeast's comfort zone (read the packaging). You should sterilize anything that will touch your boiled water after it stops boiling, including the spoon/stick with which you will mix it into your wort.
As for any good tasting tap water being ok... spend a few minutes and read this page, http://www.sensiblemole.com/watertreatment.htm. It is from an all grain site, but the two techniques it offers are universally applicable.