  We present Halpha3 (acronym for Halpha-alpha-alpha), an Halpha narrow-band
imaging survey of ~400 galaxies selected from the HI Arecibo Legacy Fast ALFA
Survey (ALFALFA) in the Local Supercluster, including the Virgo cluster. By
using hydrogen recombination lines as a tracer of recent star formation, we aim
to investigate the relationships between atomic neutral gas and newly formed
stars in different environments (cluster and field), morphological types
(spirals and dwarfs), and over a wide range of stellar masses (~10^7.5-10^11.5
Msun). We image in Halpha+[NII] all the galaxies that contain more than 10^7
Msun of neutral atomic hydrogen in the sky region 11^h < R.A. <16^h 4^o < Dec.
<16^o; 350< cz <2000 km/s using the San Pedro Martir 2m telescope. This survey
provides a complete census of the star formation in HI rich galaxies of the
local universe. We present the properties of the galaxy sample, together with
Halpha fluxes and equivalent widths. We find an excellent agreement between the
fluxes determined from our images in apertures of 3 arcsec diameter and the
fluxes derived from the SDSS spectral database. From the Halpha fluxes
corrected for galactic and internal extinction and for [NII] contamination we
derive the global star formation rates (SFRs).
