There are two answers to this depending on where you are looking!
Reading to Basingstoke services now mainly use two 'proper' 3 car 150s, these are the original Class 150 prototypes (150001/002) that were built as three car trains without interconnecting gangways.
There are also two other 3 car 150 units (renumbered as 150921/927) working in the west of the area - but these are made up from 2 car units in a similar way to the 3 car 158s, ie there is a 'spare cab' in the consist.
The background to the hybrid three car units with FGW▸ is that there were two carriages that lost their original 'other halves' to accident write-offs, and because neither had a toilet they could not be made up into a two car unit. When the DfT» transferred most of the LM▸ fleet to FGW and Northern the units had mostly been running around in three car hybrid formations, but they were all reformed back into their original 2 car status and the 2 orphaned carriages mentioned earlier were initially unallocated until FGW took them on.
Paul
I find it strange that 2 class 150 half units could NOT be made into a two car unit because of the lack of a Toilet!
After all is a Class 153 (which is a single car) taken out of service if the Toilet is defective!
Would it be that the Emergency rescue equipment locker on a Class 150 is carried in the carriage that also happens to hold the toilet so that a Twin class 150 that had no toilet carriage would have no EMERGENCY equipment?
Makes much more sense to Smokey