It could be a bunch of things........from form, to equipment, to something as stupid as 3 arrows that have the wrong field points?
A good start would be to eliminate the arrow theory by numbering them (a small number in the corner of a vane) That way if #' s 4 and 7 always hit low you can suspect the arrow.
Start there, but it actually sounds like a form problem........maybe an inconsistent anchor point, dropping the bow arm etc.