The present invention is directed to both a method for ensuring optimal conditions for separation, and a separator device for extraction of water, oil and gas from a mixture of water-oil-gas flowing in a straight pipe or straight channel. A separator comprises a certain length of pipe with diameter larger than or equal an inlet pipe and several inclined and profiled holes along the separator pipe wall. Holes in the pipe wall at the bottom of the pipe are provided for extraction of water, and one or more groups of inclined holes in the pipe wall at the top of the pipe are provided for extraction of oil and possible gas. Water and oil/gas are separated such that dispersed flow in the inlet is reduced to velocity equal or less than that of the separated flow at the outlet of the separator.