If you are in an area where the pH is naturally low there is little you can do to seriously change things unless you add limestone. If you struggle to maintain low pH, then be wary. That said, 3 yr old manure sounds safe to me. Maybe someone with actual experience will say differently.

I've often used it and as long as it isn't soggy it will be fine. Not all stable waste is equal and sometimes it can be heavy and wet which is not good for blueberries. Other times, if it has broken down in aerobic conditions particularly, it can be kind of foamy and light which is great.

Blueberry roots need a lot of air and yet their soil must be constantly moist, at least while they are establishing. Mulch is very useful to accomplish both issues.

I added some peat moss when i planted mine and lay the woodchips on very heavy...mine have looked very nice.

Personally, I would go ahead and use the "three year old horse manure" on our blueberry shrubs. We have alkaline clay soil, and I check soil pH twice a year, in the early spring, and again in the fall. I am using agricultural sulfur to lower soil pH as necessary. I would expect composted horse manure to be slightly acidic.

I would use it. The shavings are probably pine, so should be slightly acidic. At any rate, stable manure should not raise the pH and it certainly improves the soil.

I wouldn't put it on too thick because it tends to be solid and heavy when it is wet. (But I'd for sure take all the aged manure they'd give me)