Got a question and don't have a code book....

A 3" P.V.C.conduit is ran from under ground transformers
to electric meter packs with 150 breakers. All meter packs
are main lug, no main breaker. Some meter packs contain six
meters and some five, some four. 120/240 volt residential.
I know I can parallel the Pratt 250-250-4/0 alu. in a 3"
p.v.c. that big enough to handle the load for
the five and six meter packs ?
6x 150= 900 total amps for the six banger @ 45% = 405
amps ? can't remember how much to derate...
Five meter pack may work with 250...just not sure about the six ... thanks....