The harass unit for holding 3 Mavericks on one hardpoint is no longer used by US forces anymore because of weapon safety and aircraft safety concerns. The 3 unit rack for Mavericks also increases drag dramatically as well.
However I do believe the USAF, USN, and USMC still use sometimes the 2 unit rack for Mavericks in certain circumstances. Most of the time they just strap on one Maverick per hardpoint.
With the development of effective cluster munitions, JDAM bombs, and effective smart weapons like the JSOW-A are reducing the necessity of using Mavericks on the battlefield.
That's my analysis of it, but that's a fair consenus of most modern aircraft weapons gurus I believe.