Aaron,
Duplexor is a nomenclature intended to mean talking and listening simultaneously where as a diplexer is to frequency separated combination of two bands. A duplexor in cell phones for example is a type of diplexor. Combining a type of low pass and a highpass to create a three port is a diplexor where the common port see the response of both filters and one port observes the response of the low pass and the other the highpass. A triplexor is to combine three filters separated in frequency.
Yes, its a four port with one common, one low pass only for match, one high-pass only for match and one band-pass as the desired.
The link you put in your post is a type of triplexor but it is made up of three bandpass filters where as what I am talking about is wider in bandwidth. Consider this link to demonstrate.
http://productfinder.pulseeng.com/product/C6178NLclick on data sheet and look at the return loss characteristics.
I hope that clarifies
http://rfcooltools.com