AKA gray asphalt wrote:
I need a shelf to hold 3 CRT monitors above my existing desk. It needs to
extend about 36" from a normal house wall and hold the weight of the
monitors. Is that enough to go on? The monitors are 2 19" and 1 21 inch. I
guess they weigh about 40 lbs each.

I think you're underestimating the weight.

My 19" monitors are 50 lbs, and my 21" is 80 lbs.

The best would be aircraft cable up to the ceiling to support the front
edge of the shelf.

Barring that, you'd want substantial diagonal braces above or below the
shelf. That much weight 3' out from the wall is going to put a lot of
torque onto the supports.

Chris