I file married filing jointly for tax return this year. My income is 3 times lower than last year. I paid estimated tax all year. Why my tax refund is so low

If you filed jointly, your income and tax is computed on the combined amount so if your spouse's income/withholding was not sufficient your estimates may have been applied to the tax owed.  Also, perhaps if you were self-employed this year but not last year, your estimates are also applied to pay the self-employment tax in addition to the income tax.