Hilary Duff -- Bikini Ready ... 3 Months After Baby

Jessica Simpson should probably take note ... 'cause Hilary Duff has already bounced back to bikini form ... just 3 months after giving birth.

The 24-year-old actress -- who's married to retired hockey star Mike Comrie -- was hangin' by a pool in Mexico ... relaxing with her new son Luca Cruz, who was just born on March 20.

It appears Duff has already dropped most of the baby weight ... and she doesn't even have a $4 million deal with a weight loss company.

Nice work.