I'd say the biggest mistake in going for a back 3 was not having players in it that could move forward if it wasn't working. ie having Dieng in the 3 so he can go central mid if we go back to a back 4. Or Jones at RWB so he can move forward and then Kusunga to RB.
Obviously wouldn't have been the best place to start Jones, that was just an example, but we had 5 defenders starting with none of them capable of playing in midfield should a change be required.