If the income from the third job will be erratic, and is not full-time, you could select in the estimator that you currently hold the job, but only for part of the year, and figure the total amount you expect to earn.
You could also choose the TurboTax W-4 Estimator if you find it easier to use.
Remember that these tools are just that - estimating tools. You can always adjust your withholding during the year if something changes.
See also this TurboTax tips article for more information on this topic.
@JohnduBois
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