2 and 4 way couplers / combiners are commercially available  but not 3 way models! I needed one for stacking 3 YAGI's on 2m so had to build one myself.
For matching 3 x 50 Ohm antenna's (= 16,7 Ohm) to 50 Ohm a quarter wave section with impedance of 28,9 Ohm is required. When working with these low impedances, the matching must be made very accurate, otherwise high SWR is the result !
If you want to work with Nstandard connectors (we don't want to go for less !) , you will have to find N chassis connectors with small flange (easy to find on HAM flea markets). These will nicely fit on a square aluminum profile of 20x20 mm.
When choosing your profile, be sure it is of 'conductive' aluminum ! No joke, but some profiles are passivated / chemically surface treated, and are totally not conductive... Measure it with an Ohmmeter to be sure....
For a characteristic impedance of 29 Ohm , the ratio (for a square coax) is exactly 1.50  see calculation below with ''AppCad'' of AGILENT Technologies (BTW, a very useful freeware)
For a square profile of 20mm, the inner dimension is 16,7 mm , thus the transmission line must have a diameter of 11,1 mm... not a very convenient value !
For a square profile of 15mm, the inner dimension is 12,0 mm , thus the transmission line must have a diameter of 8 mm... this is very easy to find !

Calculations with AppCAD 
The solution is to use a square profile of 15x15 INSIDE the 20x20
profile ! The inner profile must not have the full length , but it is
enough that it is fitting between the outer Nconnectors and mechanically kept
in place (I used on both ends 3 pcs M3 screws through the outer
tube, so the 15mm section is compressed against the 20mm profile) then removed the exceeding part of screws).
The total length of 20x20mm profile is 517 mm + 2x 10 mm for the 'ends' = total 537 mm (this is not very critical).
The Nconnectors can be mounted with self tapping screws or M3 screws (UMBRACO preferred). Use some 2 component epoxy glue to make a watertight joint between the connector and the profile.
The transmission line is a 8mm brass tube (you can find it in any hobby shop). Be sure it is well centered in the 15 mm section. Result : testing with 3 dummy loads connected, SWR 1:1 from 140150 Mhz !
The feed 

The 3 way head 