Masters of Camouflage
ASTRA MILITARUM WALKER and REGIMENT models from your army have the Benefit of Cover. While such a model has the Benefit of Cover for any other reason (e.g. because it is wholly within a RUIN), improve the Save characteristic of that model by 1 (to a maximum of 3+).
It would be 3+. When you have this rule and are getting the benefit of cover from a ruin the save
characteristic of a model increases by one up to 3+.
While getting the benefit of cover you add 1 to
armor saving throws (unless the model is 3+ and the attack has an
AP of 0).
So a model with a save of 3+ is hit with an attack with an
AP of -1 deceasing the armor saving throw by one and the benefit of cover then increases the saving throw by one evening out at 3+.