According to Common Core, this is incorrect. It should be:
Why "5+5+5=15" is wrong under the Common Core - Business Insider
They say the second one is correct because it's better to think of it as "5 groups of 3," rather than "3 groups of 5." This approach makes it easier when the student moves on to calculus with multiple variables.
I read somewhere Common Core math is designed to get students to think of math as more "fluid," and that will help them think more abstractly and foster innovation.
Is this all a bunch of malarkey, or do the Common Core people have good intentions (e.g., improve testing on STEM subjects) gone wrong?