It is thought that the kittens were dumped by an owner struggling to look after them amid the cost of living crisis.
The reusable shopping bag was covered in illustrations from the Harry Potter series – including wands, owls and the Hogwarts logo.
They were discovered beside a road in Birmingham.
A member of the public found the kittens and contacted the RSPCA.
The adorable trio were named after the book and film series’ main characters.
It is unclear if the kittens will be split up when they are ready for new homes.
RSPCA Inspector Jonny Wood said: “Thankfully these little guys were found and have been given the care and attention they deserve.
“But we’re pleading with people not to dump their pets.
“It is never the answer.
“We appreciate the financial difficulties some people are experiencing right now.
“However abandoning your pet, or not seeking appropriate veterinary help if it’s needed, is never acceptable.”
The RSPCA also revealed that they received 1,822 reports to its emergency helpline about an animal being abandoned in May.