Okay then it is fairly simple.
Run a cable from the SLNB to the room where you want the second decoder located. The cable should be connected to port B on the SLNB, assuming the existing decoder is connected to port A.
Once the decoders are connected you need to configure the network settings.
I presume the Explora 1 is set to use UBs 0,1,2, which is the decoder 1 settings?
Set the Explora 3A to make use of decoder 2 settings as per the installation wizard.
Next hurdle is the HB. you will need to get a Diplexer to install behind the Explora 1 so that it will be able to receive HB from the Explora 3 via the SLNB cabling.
You will need to sort all this out before you contact MC to setup XV.
Do you have a diplexer?
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
What LNB is installed in the antenna, or if in a complex what LNB connections do you have available?