If your Form 26AS only shows TDS for one month of salary, despite working for three, you need to address this with your employer. Ensure they have filed the TDS return for all three months and deposited the deducted amount. You can only claim TDS once it reflects in your Form 26AS. Filing your Income Tax Return (ITR) without the full TDS amount shown could lead to a tax credit mismatch. If your employer hasn't filed, you may need to wait or consider declaring the income in the next financial year, though claiming past TDS in future returns isn't guaranteed.