You will need to use a constructed list. The constructed list's parent list should be the list used for C3. The instructions for the constructed list depend on whether C3 is multi-select.
Single-select:
Begin Unverified Perl
my $c3 = GETVALUE('C3');
if ($c3 == 1 || $c3 == 3) {
ADD('list1', 1);
}
if ($c3 == 2 || $c3 == 3) {
ADD('list1', 2);
}
ADD('list1', 3, 6);
End Unverified
Multi-select:
Begin Unverified Perl
if (GETVALUE('C3_1') || GETVALUE('C3_3')) {
ADD('list1', 1);
}
if (GETVALUE('C3_2') || GETVALUE('C3_3')) {
ADD('list1', 2);
}
ADD('list1', 3, 6);
End Unverified
"list1" must be replaced with the name of the parent list.
Finally set C6 to use this list.