Why did my third coat of polyurethane finish "buckle"?
It is very unusual for Urethane to Buckle or alligator skin when applied over urethane. The reason for this type of failure is usually a contaminate on the surface. Grease, oil, wax etc. Did you use the same type of urethane from the same lot? After sanding, did you clean the dust off with a cloth that may have had some contaminates on it? I always use a clean cloth very slightly damp with water, or a tact cloth, never use mineral spirits. Give it a while to dry completely and put on the next coat.  The only other problem could have been that the urethane was not mixed well. Always stir, never shake urethane.  Don't mix water base urethane with oil base.