Unfortunately, from what I can tell, Luno is a "centralised" exchange service... So, you have no way to directly access the private keys for your "deposit" address. It is controlled by Luno.

You will need to contact their support and see if they can assist in recovering your coins. Note that some exchanges have a "minimum" value for recovery or charge a fee for this... and some just refuse to do it. I cannot find anything on their website regarding Luno's policies for "crosschain" coin recovery

It would be better if you started a new thread and explained your problem with more details. What are you trying to do and what do you mean with "how to u use the core address"?
An address that starts with 3 is most of the times not a multisignature address. It's usually a P2SH or Nested SegWit address. What problems do you have with your "3" address?   

since Electrum was mentioned above, i have to say there is no option to import a single multi-signature (private keys and redeem script) into Electrum. your only option is if the address was derived from master keys and you import those in Electrum.
if your address starting with 3 was a multi sig then your only option is to use the core wallet.