Jesus wept - there are reserve versions of the Commando course and P Coy. If you have a justification for going on them, you will be able to apply.
6 Rifles are the reserve Bn for 1 Rifles, which will sit within 160 Bde not 3 Cdo Bde and therefore have no Cdo role or requirement to attend the Cdo course. Therefore, why would you expect your unit to pay for you to attend a course you don't need?