Central heating/cooling system heating unevenly
The "Registers" on the blowy holes are adjustable down to fully closed. The Grills on the sucky holes are not adjustable. You should find that the middle room's Register can be almost completely closed in order to maintain an even temperature equal to the other rooms.

Unfortunately, this adjustment or setting only applies to heating as you discovered upon switching from cooling. You'd need to either manually set the middle room's Register each heating &amp;/or heating season or have a programmable &amp; motorized External Register or Internal Damper installed to do it for you.