The refund amount that you see in TurboTax doesn't really mean anything until you have entered all your W-2s, because your actual refund is based on your total income and the total amount of tax that was withheld.
Each W-2 has an amount of income and also an amount of tax withheld. Depending on the amount of tax withheld in relation to the amount of income, each W-2 that you enter could make your refund higher or lower. And how your refund changes as you enter each W-2 could vary depending on what order you enter them in.
Also, your standard deduction reduces your taxable income. But you only get one standard deduction. The first W-2 that you enter probably uses up the entire standard deduction. So the income on the additional W-2s is fully taxable. Adding more taxable income, with no more deductions, reduces your refund.
It could be that your other two employers did not withhold enough taxes for you.
If you are still employed by either of them, check your W-4 Withholding and submit a new form.
Click the link for a W-4 Withholding Calculator to help you.
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That's common. It just looks that way because you put them in as separate W2s and saw the tax due change in between them. If it all was on one W2 you would get the same answer. And each job was withholding like it was your only job.
Because you only get one standard deduction no matter how many W2s you put in. Turbo Tax starts out by giving you the Standard Deduction. You entered more income when you entered the second W2 but you didn't enter more deductions. And each job only withheld taxes like it was your only job for the year. You might want to adjust your withholding. Also as you add more income you might not be getting as many credits as before like the EIC credit. And it might have been giving you the EIC credit or some other credit until you went over.
See this FAQ