I just bought a 43 year old house and it doesn't have any soffit vents, which makes the attic extremely hot. The soffits are covered with stucco (like the rest of the house) and I was hoping to install 3" round vents all along the house. I would need to install about 20 of these. I have tried a standard hole cutting bit on a hammer drill and it cut through great, but of course wears the bit down after only one hole.

Can anyone recommend a bit to do this, probably a diamond carbide one I would imagine, but i want to make sure I get one that can do the whole project if possible.

 

You need a 3" core bit. Similar to those pictured here. They are not cheap. You will want to ensure that drilling the holes is actually going to give you ventilation, and that the soffit is not blocked off from the attic rafters along the top plate. Sometimes there are obstructions that would block the passage of air.

 

I cut many holes in stucco using a very simple method.

Using a round object the size of the hole wanted draw circle with a marker.
Take a 3/16" or so masonry bit and drill a series of small holes around the perimeter of the mark and a few in the center then chisel out the center.
A hole saw will then drill through the wood.

For as many as I do the few minutes each hole takes makes it not worth spending the money on a core bit.
I can usually use a cordless drill on high speed for the drilling.