Twitter / You can schedule your tweets in advance using various apps, such as Buffer or Hootsuite.
Facebook / Did you know you can schedule your facebook page posts too? Simply start typing your post and under the 'Publish' drop down menu, choose 'Schedule' and away you go! You can also backdate posts too.
Instagram / Posted up an image you're now not so keen on? Some Instagram phone apps won't let you delete the posts. I did a bit of digging around and found that the 6tag Instagram app does, phew!