Cutting boards to fit between each joist is part of the trade. The same result may be obtained by fastening a continuous board (2X4) to the bottom of each successive joist. Your joists are clearly "overspanned". Structural failure is not a concern, however occupant comfort has been compromised. If your home is 3yrs. old, I would contact the builder first. There are certain codes that he must follow for new home construction. Blocking may help, but the odds are not in your favor.