It is OK to just add a 100 amp disconnect for the shed. You are allowed up to six disconnects per service (230.71). The riser or lateral (and the meter enclosure) would need to be adequate for 300 amps if the load calculations warranted it, but, you could then tap off to each disconnect with 200 amp and 100 amp taps for the two buildings. What feeds the shed?
By the way, each building needs its own disconnect.
"VI. Service Equipment — Disconnecting Means
Means shall be provided to disconnect all
conductors in a building or other structure from the service entrance conductors.
(A) Location. The service disconnecting means shall be installed in accordance with 230.70(A)(1), (2), and (3).
(1) Readily Accessible Location. The service disconnecting means shall be installed at a readily accessible location either outside of a building or structure or inside nearest the point of entrance of the service conductors."
I infer from your description that the service conductors stop at the shed, and feeders will feed each building.
"225.31 Disconnecting Means.
Means shall be provided for disconnecting all ungrounded conductors that supply or pass through the building or structure.
The disconnecting means shall be installed either inside or outside of the building or structure served or where the conductors pass through the building or structure. The disconnecting means shall be at a readily accessible location nearest the point of entrance of the conductors."
How close is that generator shed? Sounds like you need additional disconnects at each building.