No dancing @ Sheva Brachos

Harav Moishe Sternbuch Shlita writes, 

"Even though there is no Chiyuv to make 

Sheva Brochos every day but if you do make

it you are performing a Mitzvah Kiyumis.

For this reason we may make 

Sheva Brochos in the three weeks.
Simchas Chosson V'Kallah is a Mitzvah. 

Refraining from Simcha in

the three weeks is only a Minhag. 

The Minhag applies only to Simcha

of Reshus and not Simcha Shel Mitzvah".  

R. M. S. continues that the Steipler zt"l 

held, being there is no Mokor  for

dancing at a Sheva Brochos, therefore 

you should refrain from dancing

during the three weeks