3-way corner fitting
3-way corner fittings are typically used to split an end point into 3 directions. The 3-way corner fitting easily connects three posts / beams.
This coupler from Thibo Online has a diameter of 60/60/60 mm. This means that it can be used to connect 3 tubes with a diameter of 60 mm together.
They are available in various versions and RAL colors: aluminum, 6005 (moss green), 6009 (firgreen), 7016 (anthracite gray) and 9005 (jet black).