Eric Fidler has the scoop at Greater Greater Washington:
The Office of Planning submitted the draft amendment for the Southeast Federal Center Overlay Zone, which covers about two blocks west of the Navy Yard. The proposal would let developers make buildings taller and with a higher Floor Area Ratio (FAR) as long as that 8% of the “bonus” area were three-bedroom units.
The Office of Planning hopes that they can add more housing for families by adding regulations that create incentives to build more large units than they otherwise would.

OP will probably be successful in adding additional three-bedroom units to the market, but it’s unlikely that the new regulations will achieve their stated goal.
As I’ve noted before, approximately 57% of D.C. households consist of a solitary person occupying a unit or home. However, our current housing stock doesn’t reflect our demographics.
Lots of D.C.’s young people live in what were designed as single-family units in a shared setting with other adults. A three-bedroom is substantially cheaper than three one bedroom (or studio) apartments in almost every case, so they can save money by teaming up for a lease. Families will still have to compete with people looking to live with roommates when seeking housing, and that fact won’t change because OP wants it to.
If the Office of Planning wants to create more places for families to live, it should allow developers to build what makes sense for each project. This probably means building smaller units for the time being, but each new unit is one fewer person competing with families for existing larger units.