If you visualize four layers at the eave, where the metal nosing is, it is actually the starter course and first course. If there are four separate layers at the eave that would actually be two roofs. The only variable would be if a new piece of nosing was installed OVER an existing roof, when installing another roof.
I do not recommend you recover a third time for a few reasons. One, the weight. Two, the quality of the installation will be affected by the likely deteriorated condition of the material you will be going over. Three, after this many years it would be wise to examine the roof sheathing.
Replace any broken or dry rotted sheathing and if it's one by six "solid sheathing", consider skinning over your roof deck with 1/2 in ply or oriented strand board sheathing. (You can go right over the existing 1X6 sheathing without having to remove it.) It will give you a more consistent nailing surface without the occasional "nail in the gap" effect, common when going over 1X6 solid roof sheathing, and provide the shear strength afforded by a plywood sheathed roof deck.
Was a roofer for over forty years and although you can probably get away with a third re-cover, in my experience, it is not a good idea at all.