Is he married?
The two dependents he already claims - how old are they? Are they related to him (if so, HOW?)?
The three potential dependents - do they live with him all year? Are they related to him (if so, HOW?)? How old are they?
I do need a bit more information. How much federal tax was withheld from his W-2 so far this year. How many more paychecks will he receive this year?
Do you have any income?
I threw some round-about numbers into tax software to see how the refund would change.
With the estimated figures you gave me, I came up with: With 2 dependents (his children), filing Head of Household, he would get back approximately $10,306.
Filing a married joint return with you, claiming 4 dependents (your 2 children and his 2 children), you will receive about $18,281.
Please keep in mind, I am not saying your refund will be $18,281. I am showing you, without taking any possible education expenses (for you or your daughter), his refund will increase just about $8,000 if you two file a joint return and claim 4 dependents.
Please let me know if I can assist you further.