The W-4 form is 4 pages with instructions and worksheets, not just the front page you turn in to your employer. You will need to analyze your situation and then probably turn in new W-4 forms to all three jobs (especially if you have not used the new form at your other jobs yet.)
Either use the worksheets and instructions on all pages of the W-4 to account for your 3 jobs, your deductions, and any dependents; or use the IRS withholding calculator as mentioned in the other answer.
