it's quiet common for engines to only have 3 mounting bolts in certain
applications, b&s usually end up that way if they wanted 4 bolts then
the chassis would be made to accomodate that. and with 3 bolts firmly
fitted nothng can happen well in all my years of mower mechanicing
i've never seen any problems.

are these 3 bolt fixings at the 120 degree, 120 degree and 120 degree? and
is it common to have 90, 90 and 180? Seems odd to me.
Mike