But we have to be clear here. You cannot bridge them while still connected to the street 
The bridging would be done strictly while islanded.
It is quite common where people use a single-phase generator to backup a 3-phase house. See @midnightcaller 's post here, for one wired exactly like that. You’ll note the bottom of the changeover has a blue and yellow wire connecting the phases together, but the top (the street side) is proper three phase.