It's my third day on twitter and I only have 13 followers?

[Rebecca]

there's no problem at all. This is completely normal. But it differs for some people. Some might have 100 followers in a week. some may have only 50 after a month. Try to follow more more who say "I follow back" in their bio. Or look for people with roughly the same amount of followers as the people they follow.
Also don't be afraid of asking people for a 'followback'. most of the tine they will. Or youu could ask them for a shoutout. Where they send out a tweet asking their followers to follow YOU.

Hope this helps. x