We investigate the star formation main sequence (MS; SFR-M*) down to 10^8-9^M_{sun}_ using a sample of 34061 newly discovered ultrafaint (27mag<~i<~30mag) galaxies at 1<z<3 detected in the GOODS-N field. Virtually none of these galaxies are contained in previous public catalogs, effectively doubling the number of known sources in the field. The sample was constructed by stacking the optical broadband observations taken by the HST/GOODS-CANDELS surveys, as well as the 25 ultradeep medium-band images gathered by the GTC/SHARDS project. Our sources are faint (average observed magnitudes <i>~28.2 and <H>~27.9mag), blue (UV slope <{beta}>~-1.9), star-forming (rest-frame colors <U-V>~0.10 and <V-J>~0.17mag) galaxies. These observational characteristics are identified with young (mass-weighted age <t_M-w_>~0.014Gyr) stellar populations subject to low attenuations (<A(V)>~0.30mag). Our sample allows us to probe the MS down to 10^8.0^M_{sun}_ at z=1 and 10^8.5^M_{sun}_ at z=3, around 0.6dex deeper than previous analyses. In the low-mass galaxy regime, we find an average value for the slope of 0.97 at 1<z<2 and 1.12 at 2<z<3. Nearly 60% of our sample presents stellar masses in the range 10^6-8^M_{sun}_ at 1<z<G 3. If the slope of the MS remained constant in this regime, the sources populating the low-mass tail of our sample would qualify as starburst galaxies.