It means you will receive a $3,000 deduction from your state income when computing your tax liability. Here's an example of how this could impact your state taxes. You have $50,000 in income before the deduction and say your state has a 5% tax rate. Instead of paying taxes on that full $50,000, you get to subtract $3,000 before the 5% tax rate is applied. So, your tax due will be $2,350 ($47,000 * 5%) instead of $2,500 ($50,000 *5%). So, the deduction saved you $150 in tax due.
What it does not mean is that you will get a full $3,000 refund to you. Your actual savings will only be a small portion of the deduction. And, its possible you may not see any impact if you already have other deductions or credits that have reduced your tax due to zero.