While I am not licensed in WI, I would be stunned to learn the law is different in this regard. You are acting only as agent of your mother and as such are responsible to repay the credit card liability only out of her assets, to the extent possible.
I assume the $3K balance was incurred by your mother before going on Medicaid. Did you personally make any of the purchases for her? Were any of them for your benefit rather than hers? If all of the purchases were made by her or for her benefit, I do not see how you would be personally liable for them. They can pursue collection from your mother, but she is insolvent except for the $45 monthly personal allowance from Medicaid. Eventually, Chase will have to submit a claim against her estate, which may or may not be solvent at the time.
Theoretically, your mother could file for bankruptcy to stop collection action with the automatic stay and discharge the debt. I'm not sure it's worth the effort, however.
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