What role does a third party insurance company play in providing coverage? What do they cover after primary and secondary coverage?

Is this a Medicare Advantage Plan? A Catastrophic plan?

Is your Mom in Longterm care? Is she getting Medicaid to pay for that care? If getting Medicaid to pay for her care Medicaid for health comes with it.

In my State having a 3rd coverage is not needed. I dropped Moms secondary once she was on Medicaid. But from members of this forum, I have found that some states require that you keep the secondary you have. But if they require this, the premium comes out of the person's SS receiving care. The family does not pay nor can the person in care on Medicaid because they have no money. So if you are being asked to keep the secondary your Mom had previously, ask them how is the premium to be paid.

When there is a 3 suppliment involved it works like this lets say its Blue Cross:

First Medicare is billed and they pay their share of the bill, then Blue Cross, they usually pay the balance but if not, Medicaid picks up the rest.

Do you mean tertiary insurance, as in your mom has primary and secondary coverage AND a third coverage?

Does mom have Medicare Part A and B AND another policy?

You say you are being asked to put mom on Medicaid. Is that the third insurance you are talking about?

Go to Medicare.gov for more information. Medicare is ALWAYS primary. Medicaid is the payer of last resort.