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The Hubble Space Telescope has enabled us to establish a broad picture of how present day galaxies came to be. We propose to extend this legacy into unprecedented new territory - degree-scale infrared imaging and spectroscopy - that will allow us for the first time to study the physical processes that shape the behemoths of the universedoublePoint the most massive comma highly-star-forming comma rare galaxies up to z=3.5. Using the d_commadrift and shiftd_comma (DASH) technique comma developed in Cycle 23 comma we propose to survey the entire 1.7 sq. deg. COSMOS field in WFC3-H(F160W) and WFC3-G141 in order to map the build up of stellar structure via >1000 resolved H-alpha maps comma carry out a statistically robust census of virgul450 M>10elev11 M_sun galaxies comma identify up to 100 massive major mergers to constrain theoretical models comma determine emission line redshifts for virgul37 comma 000 z>0.7 galaxies and provide the most accurate predictions on the bias and number density of emission line galaxies prior to Euclid and WFIRST. 3D-DASH will create a wide shallow tier in the infrared d_commawedding caked_comma of extragalactic surveys comma increasing the area observed with infrared grism spectroscopy by almost an order of magnitude and adding HST-resolution infrared imaging to COSMOS comma one of the most actively studied fields. This program will fill in an important gap in HST extragalactic surveys in the pre-JWST era and is unlikely to be surpassed in sensitivity comma area and resolution until the launch of WFIRST.