I recommend Pin the tail on the Donkey, a pinata, Wii Bowling, Go Fish, water balloon toss (if you can be outside), Bingo, decorate your own cupcakes, Red Rover, Duck Duck Goose, Hot Potato, and Red Light Green Light.
For 5 year old unisex triplets, you really can't go wrong with "pin the tail on the donkey", "red light, green light", "simon says" and "hot potato". Another fun activity would be to hit a pinata full of candy, many kids love to do that on their birthdays! If all else fails, "hide and seek" or "tag" should work!
Pinatas are typically fun for this age group and are popular among both boys and girls. Pin the Tail on the Donkey is also a good choice. Musical chairs could be good as long as there is a proper amount of supervision to prevent accidents and fights. If the weather is warm you could play numerous water games.
Pass the Parcel.... wrap up a prize. Wrap with another piece of paper enclosing a small gift (sweets are ideal. Repeat as much as you like. Kids sit in a circle. Play music as they pass parcel round. Stop music at intervals. Child holding it removes a layer of paper.