Fish tank air hose connectors: 3-way, right angle.

thingiverse

These are inch models but when imported to the slicer Simplify3D, which is mm only, the longest dimension on the 3-way connector is 22.60mm. I have printed these for my aquariums on a Dremel 3D20, which is a cheaper printer with an unheated bed. They can be printed in a few minutes, without support material, make sure you print with a "raft" (layer of plastic below model). This is because there are only a few narrow contact points without support material, so unless a raft is used they will probably pop off the bed. I found when I printed at "medium" quality in Simplify3D I got the best results. High quality made the contact with the raft too small. At low quality, I got too much air leakage through the walls of the model. You will see that there are double flanges, having two rings on each end. This gives a very tight grip and keeps the air hose from falling off, even if it is tugged on. If you have to join many hoses, you will do your fingers a favor by using a lighter. The tip of the flame from a lighter can warm the end of the hose for three or four seconds, then it will slide onto the connector. You can use the connector without warming up the air hose and it works fine, you just have to twist as you push it on. If you have to join 20 hoses, please use a lighter, or a cup of very hot water from the microwave, and save yourself tired hands.