Re: Merc 3.0 L Therostate removal
That's what happens when people post questions and say 3.0L and never give a year or Serial Number. The systems change from year to year.
That sleeve is being held in by rust. It may even break getting it out. But, it does have to come out to get the thermostat out.
Here is a picture out of manual #26. The part isn't even identified or named, but it's there.