ci_cmake: Add missing version 3 for RD-N2 build

A recent change [1] for RD-N2 platform introduced a new variant
3 but the ci build was not added.

Add it now.

[1] ebc5d50a

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I12afacc115f97ce7126ec3be0eb186beb26ca3c0