The limit on the total amount of gas that can be used in a block (the Block Gas Limit), by all transactions, is currently ~6.7M, so this transaction will currently never succeed.
You can't increase this total limit on the amount of gas used in a block, only the miners can agree on what this should be.
If you were somehow able to get around Mist preventing you from sending this transaction and you set the gas limit on the transaction (the total amount that of gas that you are willing to pay for) to 3M then it would run out of gas before it completes, the transaction will fail and you would have effectively have burnt the Ether / given the miner the Ether for nothing.
I recommend you rethink about what you are trying to do with Ethereum; try to remove storage of data that does not need to be on the blockchain, break the code into smaller functions etc.
for (uint k = 0; k < 3000; k++ ){ new_value = k; }and my gas was enough for 2000 steps, which value will I have innew_value? Default value or 1999 ? – sp41mer Oct 04 '17 at 19:39