Am I eligible to receive extra social security benefits having served on active duty from Feb. 1956 to Feb. 1959?
Not necessarily. What you would be eligible for are military wage credits (MWC) and deemed military wages (DMW) for that period of time. Those extra wage credits could then be used in the calculation of your retirement benefit rate, which may or may not increase your rate depending on how much you earned in other years.
For more information on MWCs refer to this section of Social Security's Handbook: https://www.ssa.gov/OP_Home/handbook/handbook.09/handbook-0954.html. Information on DMWs can be found in this section: https://www.ssa.gov/OP_Home/handbook/handbook.09/handbook-0953.html.