Why is my tax higher in 2019 than in 2018? when taxable income was $3000 higher in 2018?

Is that the tax on line 12 or the tax due to pay?  I don't see that amount on the tax table  https://www.irs.gov/pub/irs-pdf/i1040tt.pdf 

Some reasons could be you had more qualified capital gains in 2018.  Or in 2019 you have a penalty for something like the 10% Early Withdrawal Penalty on a IRA or 401K distribution.  You need to post the line numbers.

there can be many reasons. we can't see your return.  for example: 1) you have dividend income and a larger portion in 2019 is non-qualified.   2) you have schedule C or self-employment income from a partnership. since that will vary the SE tax will vary. 3) you had more tax credits on 2018 return than on your 2019 return.