A three terminal junction box can be used provided you can get all three wires into a single terminal, as the clamping screws may not have much thread holding in the connector if there are too many wires being clamped underneath them. Obviously, you would only need to use two of the three terminals, one for the three red cables, and one for the three black ones.
You may need to go for a 60Amp three way connector as used on showers and cookers etc. if your wires are too thick to fit into the 30Amp terminals.
The shape of the box does not matter, it's more a question of finding something that can accommodate the thickness of the wires and handle the current.
Alan