A modified gathering manifold is disclosed, including a sampling header coupled to each of multiple production lines of wells, and a plurality of diverters, each coupled to one of the production lines, upstream of a relief header coupled to each of the plurality of production lines and a production header associated with the manifold and coupled to each of the plurality of production lines. The sampling header receives a production fluid diverted by a diverter in the open position. The manifold also includes a three-phase separator coupled to the sampling header downstream of the plurality of diverters that separates the production fluid into crude oil, water, and gas, and detects a volume flow rate for each. A return header passes the crude oil, the water, and the gas from the three-phase separator into the production header where they are combined into a hydrocarbon fluid flow.