No they can't, the 8500s/10s and 20s have to be a 4 car minimum. On the 85xx series the driver cab units numbered in the 86xx range are unpowerd, the intermediate cars numbered in the 85xx range are the power cars. A 3 car 8500 may be possible with a bit of modifying but too messy to set up if it is possible.
In the LHB DARTs the 8100 cars are the power cars and the 8300 cars are the trailers.