This is a brass Cross Fitting for hydraulic brakes.  A common use for this brake fitting is on trailers that have hydraulic brakes on more than one axle.  This fitting is normally attached to the trailer axle and splits the main line that is coming from the master cylinder down one side of the trailer frame.  The Cross Fitting allows a short brake hose to reach the near side brake caliper or wheel cylinder and a longer hose to run the length of the axle to the other sides brake caliper or wheel cylinder.  The last opening on the cross fitting is then used for a 3' hose to jump to the next axle that has brakes  This is a high pressure fitting rated for drum or disc brakes using surge couplers or electric over hydraulic units.  The ends are brass female 3/16" inverted flare fittings that use a 3/8-24 thread.