Try:
Replace 1, 2, 3 with a comma separated list of the forums you want it to apply to. Alternatively you can use separate if blocks to give a different template to each forum.
<if in_array($mybb->input['fid'], array(1, 2, 3)) then>Replace 1, 2, 3 with a comma separated list of the forums you want it to apply to. Alternatively you can use separate if blocks to give a different template to each forum.