How can l have VA medical and Medicare/Medicaid at the same time?

Medicare is the Primary insurance the VA would be the Secondary just like anyone else that would be covered by 2 policies.
The VA should submit to Medicare and anything that is not covered by Medicare the VA will cover.
You may or may not have to pay a Co-Pay depending on the situation. (% of service connected disability according to the VA)
This might be something that you should discuss with a Patient Advocate