An RF combiner should be included in the triple resonant box because it eases impedance mismatching and hence lowers undesired RF reflections.
Therefore we should use three cables to send the RF signals to the box and then combine them in the box.
With proper terminations an RF combiner shows 50 Ohm input impedance.
But it still shows nearly 50 Ohm input impedance even if the source port is not properly terminated (i.e. non 50 Ohm termination).
This means any bad impedance mismatching on the source port can be somewhat brought close to 50 Ohm by a combiner.
The amount of deviation from 50 Ohm in the input impedance depends on the circuit configuration of the combiner as well as the termination impedance.
For example a resistive 3-way splitter shows 40 Ohm when the source port is shorten and the other ports are terminated with 50 Ohm.
Also it shows 62.5 Ohm when the source port is open and the other ports are terminated with 50 Ohm.
In this way an RF combiner eases impedance mismatching on the source port.
(RF signal transfer at the 40m)
According to the prototype test of the resonant box it will most likely have a non-50 Ohm input impedance at each modulation freqeucy.
If we install the resonant box apart from the combiner it will create RF reflections due to the mismatch (Case 1 in the diagram below)
The reflection creates standing waves which may excite higher harmonics and in the worst case it damages the RF sources.
To reduce such a reflection one thing we can do is to install the combiner as a part of the resonant box (Case 2).
It will reduce the amount of the mismatching in the input impedance of the resonant circuit and results less reflections.
A rule we should remember is that a cable always needs to be impedance matched.