  The wealth of detections of millisecond pulsars (MSPs) in $\gamma$-rays by
{\em Fermi} has spurred searches for these objects among the several
unidentified $\gamma$-ray sources. Interesting targets are a sub-class of
binary MSPs, dubbed "Black Widows" (BWs) and "Redbacks" (RBs), which are in
orbit with low-mass non-degenerate companions fully or partially ablated by
irradiation from the MSP wind. These systems can be easily missed in radio
pulsar surveys owing to the eclipse of the radio signal by the intra-binary
plasma from the ablated companion star photosphere, making them better targets
for multi-wavelength observations. We used optical and X-ray data from public
databases to carry out a systematic investigation of all the unidentified
$\gamma$-ray sources from the Fermi Large Area Telescope (LAT) Third Source
Catalog (3FGL), which have been pre-selected as likely MSP candidates according
to a machine-learning technique analysis. We tested our procedure by recovering
known binary BW/RB identifications and searched for new ones, finding possible
candidates. At the same time, we investigated previously proposed BW/RB
identifications and we ruled out one of them based upon the updated
$\gamma$-ray source coordinates.
