Question;

in how many ways can a boy arrange in a row 6 balls from 7 cricket balls, 6 tennis balls and 5 squash balls?

i thought the answer would be $\displaystyle \frac{(18)(17)(16)(15)(14)(13)}{7! 6! 5!} $ but I was wrong

I tried that fact that there are 3 unlike group of items that there are 3 ways of choosing which group to go first

but from there i am a bit lost how to tackle this question

Please can you help

thank you