1  declare
  2    Function get_min(pVal1 number:=null, pVal2 number:=null, pVal3 number:=null) return number
  3    is
  4     v_ret number;
  5    begin
  6     select min(x) into v_ret from ( /* table( ...*/
  7       select pVal1 x from dual union all
  8       select pVal2 from dual union all
  9       select pVal3 from dual);
 10     return v_ret;
 11    end;
 12  begin
 13    dbms_output.put_line('2,1,3 min '||get_min(2,1,3));
 14    dbms_output.put_line('2,null,3 min '||get_min(2,null,3));
 15    dbms_output.put_line('2,-1 min '||get_min(2,-1));
 16    dbms_output.put_line('null,null,null min '||get_min());
 17* end;
SQL> /
2,1,3 min 1
2,null,3 min 2
2,null,-1 min -1
null,null,null min