How long do you have left on the property after foreclosure after receiving a letter from an attorney stating the property has been purchased and you had 3 days to vacate does letter serve as eviction?

The notification could be construed as the right to evict depending upon state laws. The best option is to contact the law firm that sent the correspondence or obtain legal advice regarding the occupant's rights. Sorry, not knowing the state of residency a more definitive answer is not possible.