One bolt (or all three) may go all the way through, but one is likely to have a grounding/ bonding strap to the engine. Can you see such a thing? I would try soaking in PB Blaster or some such, apply again every day for at least 3, and if not, remove/ drill out the screws and remove it. At that point you can temporarily mount the housing (bolts) on a scrap board or piece of metal so you can heat/ reef on it.
