Unless you are planning to breed from your cat then it is time to take her to your vet to be neutered. If she had access to 'outside' during those 3 days she could already be pregnant. If you don't want to be responsible for the healthcare, feeding, raising & homing up to 8 kittens then phone the vet tomorrow & book her in for a spey. You can get help with the costs from a charity if you need to. I hope that you've already taken care of her other health care needs such as worming & vaccinations.
If you decide that you don't want to get her neutered then you can expect her to come into season every 3 weeks for 7-10 days at a time all through the spring & summer & possibly the autumn/ winter too.... Until she gets pregnant. If allowed to come repeatedly into season & not bred from she is at extra risk of uterine infections. Hope this helps - x.