An all section excess means just that. It is an excess (your responsibility) that you have to pay for all claims made against the policy. So if the third party claims against your Insurance, you have to pay £3k, even if the claim is below that amount.
This is pretty silly. You would be better to pay the third party for the amount of damage to their car, if the amount of claim is below £3k.
Get a complaint registered urgently with your Insurers in writing, by sending a recorded delivery letter. Tell them that the £3k all sections excess is a complete nonsense and could be viewed as unfair under the Consumer Protection from Unfair Trading Regulations 2008 act. Advise them to provide full details of the claim made against the policy, so that you can review any options that may be available. Make it clear to them, that if you cannot obtain satisfactory conclusion to your complaint, you will refer the matter to the FOS and OFT.