How to add several references under one numbered reference


I’m new here so…good luck for me :-).

I’m using the numbered style of the references, now I need to use it but I need to add several references for

the same one number,for example:

[1] ref 1…

[2] ref 2…

[3] (a) ref 3a…(b) ref3b…(c) ref 4c… etc.

how do I do that?

Thank you in advance,