Hmmmm.... Hot air heats two ways.... radiant and convection. You have some of each happening. The stove sits downstairs & heats the floors. That heat radiates upwards. The closer you are to the stove, the more radiant heat you get. Hence the center of the house is toasty & the bedrooms on each end come up a bit short on BTU's...but you knew that.
When you cut cold air returns you are asking it to make convection....and it does, to a certain extent. In order for cold air to return, that volume of air has to come from somewhere. That somewhere is the cellar door. Yes, the door makes a cold air return, but that's from door knob down, above the door knob you'll find hot air convecting it's way up stairs to replace the cold air going down the vents. So... air being lazy (like I am), it likes to take the shortest, easiest path. In #2 bedroom the duct IS that path. Of course some makes it's way down to bedroom #1, but that's a long hike. Bedroom 3 can only get convection through it's doorway. If you added a vent as far away from the door as possible, surely it will convect better than it does now. ( I like th eidea of putting the vents in the walls, near the floor.... just watch for electric wires.) I had a ranch years back & I cut an inch off the bottom of each bedroom door. That was the cold air return. The ducts I had brought hot air up from the cellar. Depending on this or that, you might find reversal of air flow, depending on how hot the cellar is, and if the bedroom doors are open or shut. If the TOP if the door was trimmed an inch, then you could well have hot air coming up the stairs, over the top of the door, and down the cold air return duct. A candle flames tells air flow pretty well... but be careful!!