Big combinations doubt

If you're creating a 3-digit number, the order of the digits definitely matters. The number '251' is different from the number '215' precisely because of the order of the digits.

From the way you've described your thought process, I'm guessing you might have studied counting from a book that teaches you to select your items first, then multiply by the possible number of arrangements of those items in the case that order matters. That's an especially confusing way to think about counting when order matters, so I wouldn't recommend learning the subject that way (if you have been studying from a source that explains counting that way, you might want to try a book that explains things more simply).

When order matters (as will always be the case when you're making numbers, words, passwords, license plates, seating arrangements, or filling labeled positions, among other situations), all you need to do is count how many choices you have for each thing, and multiply. In your digits question, you have 3 choices for the first digit, then 5 for the tens digit, and 4 for the ones digit. So the answer is (3)(5)(4) = 60.