Flat rate shippig based on quantity but stops at a certain quantity

A want to charge a flat shipping rate based on quantity, flat rate of 10 for 1 or two items, and then 15 for 3 items or more. Just that.

I’ve seen this formula for shipping classes:
10 + ( 2 * [qty] ) – A base shipping cost of $10 plus $2 for each item in the cart.

But I don’t want it to keep adding dollars for every item added. A flat rate of 10 and 15 for 3 or more, just that.

How can I achieve this? Do I need a plugin?

Thanks for the help.