What if you support 3 kids and your girlfriend and those kids are not yours so basically it like a stepdad and they lived with you

[MinhT1]

You can claim your girlfriend and the 3 children as your dependents if they pass ALL the following tests:

1. The person can't be your qualifying child or the qualifying child of any other taxpayer. 
2. The person must live with you all year as a member of your household (and your relationship must not violate local laws). 
3. The person's gross income for the year 2019 must be less than $4,200. 
4. You must provide more than half of the person's total support for the year.
5. The person is a U.S. citizen, U.S. resident alien, U.S. national.
6. The person is not a married person who files a joint return unless that joint return is filed only to claim a refund of withheld income tax or estimated tax paid.

[macuser_22]

No, it is NOT like a stepdad at all if you are not married.

You can only claim an unrelated person if they lived with you the entire year AND you paid more then half of their total support for the year.

---Tests to be a Qualifying Relative (& Unrelated Persons)---
(Must meet ALL of these tests to be a dependent)

1. The person cannot be your qualifying child or the qualifying child of any other taxpayer.

2. The person either must be related to you, or must live with you all year (all 365 days - There are exceptions for temporary absences such as school, illness, business, vacation, military service) as a member of your household.

3. The person's gross income for the year must be less than $4,200 (tax-exempt income, such as certain social security benefits, is not included in gross income)

4. You must provide more than half of the person's total support** for the year.

5. The person is not filing a joint return.

In any case, the person must be a U.S. citizen, U.S. resident alien, U.S. national, or a resident of Canada or Mexico

The above is simplified; see IRS Publication 501 for full information.
https://www.irs.gov/publications/p501#en_US_2018_publink1000220939

** Worksheet for determining support
https://www.irs.gov/publications/p501#en_US_2018_publink1000292527you the entire year (all 365 days) AND you paid more then  half of their total support for the year.

[Critter]

And claiming them as qualifying relatives only gets you the OTHER QUALIFYING RELATIVE CREDIT which is up to $500 per person and is limited to your income tax liability. You do not get the Head of Household filing status or the Earned Income Credit or the Additional Child Tax Credit.