Butter/Spray a 9x13 pan
In a large glass bowl mix Lucky Charm cereal and Lucky Charm marshmallows together.
Melt butter on low heat in a large non stick pot
Add one 10oz package of marshmallows and stir keeping on low heat until they are melted into the butter. Then add in the last 2 cups and let them melt partially.
Add the melted marshmallows to the glass bowl with the the Lucky Charms mix and stir until the cereal is fully coated.
Pour mixture into prepared pan and spread out around evenly with large spatula sprayed with non-stick spray or you can use butter.
Press mixture down into pan evenly. (Don't over press, they become too compacted and hard).
Add sprinkles immediately and press into treats.
Let treats sit for 2 hours or so and cut into preferred sizes and shapes.
Store in an airt-tight container