1. A method of encoding a signal with a three dimensional image sequence using left and right video image sequences, each of the left video images being a picture, each of the right video images being a picture, said method comprising the steps of: merging each of said right pictures with a left picture continuously to produce a plurality of merged pictures, each of merged pictures containing a right field corresponding to a right picture and a left field corresponding to a left picture; encoding a first merged picture, selected from said plurality of merged pictures, to produce an I-picture; predictively encoding a second merged picture, selected from said plurality of merged pictures, by using the right and left fields of said first merged picture to produce a P-picture; and predictively encoding a third merged picture, selected from said plurality of merged pictures and located between said first merged picture and said second merged picture, by using the fields of at least one of said first and second merged pictures to produce a B-picture, wherein each of said predictively encoding steps for the P-picture and B-picture comprises the steps of: separating left and right fields of the corresponding merged picture into a plurality of left and right 16.times.16 pixel macroblocks, respectively; calculating a first value indicative of the predictive encoding efficiency performed on each of said left and right 16.times.16 macroblocks, by using a target 16.times.16 left or right macroblock and one or more same left or right fields of said first and second merged pictures; calculating a second value indicative of the predictive encoding efficiency performed on each of said left and right 16.times.16 macroblocks, by using a target 16.times.16 left or right macroblock and one or more opposite left or right fields of said first and second merged pictures; separating each of said left 16.times.16 pixel macroblocks into a left upper and a left lower 16.times.8 macroblock, and separating each of said right 16.times.16 pixel macroblocks into a right upper and a right lower 16.times.8 macroblock; calculating a third value indicative of the predictive encoding efficiency performed on each of said left and right 16.times.16 macroblocks by using a target 16.times.8 left or right macroblock and one or more same left or right fields of said first and second merged pictures; calculating a fourth value indicative of the predictive encoding efficiency performed on each of said left and right 16.times.16 macroblocks by using a target 16.times.8 left or right macroblock and one or more opposite left or right fields of said first and second merged pictures; selecting the most efficient predictive encoding process between said first to fourth values; and predictively encoding the corresponding merged picture to produce the P-picture or B-picture, based on the most efficient of the predictive encoding processes.