We can't just start re configuring our subject properties to make them worth more ! (or because we can't find comps lol)
I assume HBU is asked and anwered, residential as exists so this is not an HBU issue, it is an evaluation issue for a subject with no recent similar exact floorplan/bedroom count recent sales.
Go back in time in subdivision, surely in years past houses with the subject 2 bedroom configuration and the same sf /model converted to 3 bedrooms sold? What was the price differences, if any, between them? That will provide support for your adjustment (or lack of one) , You also might find an older comp of the 2 bedroom and make a time adjustment.
Add support for conclusion by surveying RE agents, knowing your market re Restrain noted above.