You need to practise your technique, after a while your slapping becomes more accurate and you'll be able to hit one string at a time

The G string is more difficult to slap, it's the thinnest so it makes the thinnest sound as well when you slap it. You can slap it, but when I play slap bass I usually pop the G string instead of slapping it.

Just keep practising, it just takes a while to master.