I had three "jobs" in 2018. Two of them I have W2s for. The third I have a 1099. Do I need to file the 1099 if it is below $3000?

All income must be reported on your tax return regardless of how small the amount. If the 1099 was a 1099-MISC and it was your only income, you would have to file a tax return for it if the amount was over $400 in order to pay 15.3% self employment tax. 

All of your income must be reported on the same tax return. If you already filed a tax return without the 1099 and it was accepted, you will have to amend your return to add the 1099.

Each W-2 and 1099 Misc must be entered separately in the appropriate sections

The IRS will be getting copies of each form and will expect to see you enter all of them.