Dependent 19 year old with earned income of around 3K and dividend income of $879. Required to file own return?

No she is not required to file.   If any federal or state tax was withheld ---look at boxes 2 or 17 of her W-2---she can file to seek a refund of the tax withheld.

MY DEPENDENT HAD A JOB

If your dependent has a W-2 for his after-school job, summer job, etc. you do not include the information on your own return. You can still claim your child as a dependent on your own return.  He/she can file his own return for a refund of some of his withheld wages from boxes 2 or 17 (he won’t get back anything for Social Security or Medicare), but MUST indicate on it that he can be claimed as a dependent on someone else’s return.  (Supervise this closely or prepare it for him!)

If your dependent’s earnings were over $400 and were reported on a 1099Misc or 1099NEC then he must file a return and pay self-employment tax for Social Security and Medicare.

You might also want to use free software from the IRS Free File versions:

https://apps.irs.gov/app/freeFile/

Their income is below the minimum filing requirements.  You do not enter their income on your tax return.

If there were taxes withheld from the wages then she can file a return for a tax refund.  If she does file a tax return then make sure she indicates on the return that she can be claimed as a dependent.

No. But she can file to receive a refund for any taxes withheld. 

She needs to file if her:

• Unearned income was more than $1,300.
• Earned income was more than $14,600.
• Gross income was more than the larger of:  $1,300, or earned income (up to $14,150) plus $450.

I know you've had three experts tell you that she does not need file.  But, my opinion is that she does need to file because of the rule that applies in your seldom applicable situation (#3 highlighted below). 

You do not report her income on your return. If it has to be reported, at all, it goes on his own return. If your dependent child is under age 19 (or under 24 if a full time student), she must file a tax return for 2024 if she had any of the following:

1.   Total income (wages, salaries, taxable scholarship etc.) of more than $14,600 ($13,850 for2023).
2.  Unearned income (interest, dividends, capital gains, unemployment, taxable portion of 529 distribution) of more than $1300 ($1250 for 2023). 
3. Unearned income over $450 and gross income of more than $1300.
4.  Household employee income (e.g. baby sitting, lawn mowing) over $2600 ($14,600 if under age 18)
5.  Other self employment income over $432, including money on a form 1099-NEC

Gross income is the $3K + $879.  The $879 is the Unearned income over $450.

She has to file because tax will be accessed.  Her standard deduction is $3000 + 450 = 3450.  3879 -3450 = $429 taxable income.