411A:
B-System hydraulics is provided by the No. 2 engine, as is C-System.
Quite true.
However, if number two engine is shutdown, and the B or C hydraulic systems are not compromised, these systems can be pressurized with ATM's (ADP's for you Boeing folks), keeping these systems fully serviceable for all normal functions.
A nice arrangement.
However, should another engine fail, you would now have less bleed air to power the operating ATM's, so configuration changes need to be carefully planned for optimal operation.
As the TriStar has four independant hydraulic systems, redundancy is enhanced considerably over other three engine designs.