Will I get more of a return if I have income of 20k and I claim my 2 grown children in school under age 24? I have 1 child age 16. does having 3 make a difference?

Probably, but it will depend.  If you have an income of less than 20K, and you are filing head of household your standard deduction is $9,350.  You will be receiving an exemption for both you and your 16-year-old of $8,100. With that alone you have a subtraction from your taxable income of $17,450.

Depending on how much more income you have over that $17,450 another dependent on your return will eliminate your taxable income.  So assuming you claim all of your kids you are eligible to claim, you would have $0 taxable income so you will receive a refund of everything you paid in.

Now since your 2 older kids are in college, you may qualify for the refundable portion of the American Opportunity Tax Credit AOTC.  This could increase your refund.  So, you will likely receive a larger refund by claiming your college-age dependents.