Since the third child lives with him you could not claim that child unless he gave you permission to do so.
When you amend your return you will be claiming the two children who live with you and will file as Head of Household. As opposed to your married jointly original return you will be claiming one less child so will have one less child tax credit and if you claimed the stimulus for that child originally your return will no longer include it. If you claimed earned income credit originally it will be less on the amended return since you have one less dependent. The positive side is that what you get you keep as opposed to splitting any benefits with your husband.
