Set the distance between the bracket and saddle.
Select the top face of bracket and the top face of saddle.

Do not accept the coincident mate. Instead, add a distance mate.
Click Right
(Standard Views toolbar), then zoom to the bracket.
In the Mate pop-up toolbar:
Click Distance
.
Type 3 for Distance and press Enter.
Click
Flip Dimension
if
necessary to position the bracket below the saddle face.

Accept the mate.