Can these War Flares be used to replace existing flares with 1/8 rivets holes 3 inches on center with the supplied screws?

Usually, yes, but not always. The Gorilla flares are not pre-drilled, so you can make your own mounting xxxx spacing. If you are careful removing the rivets, the screws should fit the hole that is left. Worse case, you may need to get larger screws, if keeping the same holes is important to you.