Six diamonds is an aggressive contract that "makes" by luck. But you should have been in FIVE diamonds.
By rebidding three diamonds, East is showing about 17 points. He actually has 17 high card points, plus another three for his heart singleton. You have eight points (nine, counting your spade doubleton).
You should have taken the "force" to four diamonds. With a bare minimum for this three diamond bid, East might have passed, but he had "extras" (relative to 17), and would then bid five.
You and East have 29 points (counting distribution) and nine trumps (instead of the usual eight). Five diamonds is a comfortable contract.