63 year old PA resident draws SSI, Social Security & on Medicaid. May receive $12- $15,000 cash inheritance. How will this affect SSI, SSA & Medicaid?

It won't affect Social Security, but will impact SSI and Medicaid.  Most likely it will be counted as income in the month received and therefore be over the income limit - meaning you aren't due an SSI check for that month.  It will then count as a resource the next month.  For each month the remaining money is unspent (e.g. saved in a bank account), it will count towards the $2,000 limit.  Resources are counted on the 1st day of each month and affect the SSI check for that month.  So if you are over $2,000 on the first day of the month, you aren't due an SSI check for that entire month.