I would like to know how to start solving this problem. Please provide step by step instructions in simplified form.

You need to specify whether digits may be repeated or not - for example is 3333 an allowable number?

I suggest thinking first about the last digit, which must be a 3, 5 or 7. Hence there are three possible choices for that digit. Then the first digit: it cannot be zero, so there are either four possible choices (if digits may be repeated) or three (if not). Then consider the second and third digits - how many choices are there for each of them?

Post back with your attempt and we'll check it for you.