The 24-1Z switch will allow an Explora 3A to be connected to it. The switch will support a 3-decoder combination on the Unicable ports.
The Universal ports, if available will support older decoders. and yes you can also use a diplexer if need be.
But as Jan says provide some more detail and we will be able to confirm how the decoders should be connected. Especially, tell us if you are in a complex or not and what are the connections available to you from the MUD installation.
Octo LNB; 2 by 2x4 MS; 2 by ES 5-2; Dedicated PSU for LNB, ES's, MS's; HD PVR 4P x 2; Explora 1; DSD 660; FSM
We need more information on what you want to do to be able to answer your question. What model multi switch and which other decoders do you want to connect to the 3A
Explora 2A (primary) Explora 3A Secondary SLNB LMX501 Samsung S9 Samsung Tab