Preschool can indeed be a good thing for 3 year olds and 4 year olds because it helps them get into learning more before they begin kindergarten.
However not all kids do better in school just because they go to preschool at age 3 or 4 before starting Kindergarten however some kids do love preschool and being around other 3 and 4 year olds.
I never went to preschool as a 3 year old and I did well in kindergarten and other grades in elementary school and high school and I graduated high school just fine.
But if you think your 3 year old would do good in preschool then it could be a good idea to enroll the 3 year old in preschool.
However there's no obligation to send your 3 or 4 year old to preschool as there's no legal requirement to send your child to school until they reach the age of 5.
Some children are 5 years and 6 months of age before they go to kindergarten depending on their birthday.