It depends what state you live in and how much your new household income will be. As a single mother, you are what is called "categorically eligible" for Medicaid. That is, you are eligible because of your responsibility for three children. When you are married, you would re-apply for Medicaid and list your household income (which will include your husband's income). States that expanded Medicaid will cover all families earning less than 133% of Federal Poverty, or $38,501 a year for a family of five. States that did not expand Medicaid will have their own rules about who qualifies.
In sum, getting married may or may not cause you to lose Medicaid. Your case worker could give you the specific income levels for your state.