Why is the Antonov 225 (almost) always chocked at the third wheel?
It's a matter of procedure.  Chocking a center wheel provides the greatest defence in the unlikely case the chock is jumped.  Other wheels in the truck must also contend.  If an outside wheel is chocked and that chock is jumped away, all is lost.

No matter tipping or rocking the aircraft in extraordinary winds or loading, the center of a wheel truck is the recommended place to chock.  A chock should be snug against both sides of a single tire to prevent any sway.

Most chocks are paired together by a rope or chain which would prohibit chocking around a large truck of wheels as described in your bonus question.  What's more, chocking at either end of a truck allows more elasticity and movement between the chocks than a single wheel chocked snug.