To many variables here to be able to answer your question properly. 3 oz. Of bacon, you would need a cooking scale to measure it exactly. Are you speaking of precooked or postcooked?
Traditionally, bacon comes in 12 oz pkgs. If you are dealing with 12 oz of bacon, simply cut off a quarter of the bacon and you will have your 3 oz regardless of how thick or how many slices there are. If you have a pound of bacon (16 oz.), cut off approx 1/5 of the bacon and this will give you ~3 oz.