"Three Things" is the practice of reflecting on your day and highlighting things that made it great. This is accomplished by making a Facebook post near the end of your day that highlights the things in your day that made you happy. It's all too often easy to grumble or grouse or use Facebook to whine, but this will make you reflect on what went well that day, both at the time it happens (so you can remember it) and at the time of your post.
Good example: Promptly escaping.
Bad example: Being sent to prison by a military court for a crime you didn't commit.
Here are the official rules:
(1) Posts will begin with "Three Things I liked about today" followed by three things you liked about today.
(2) No negativity. Negativity is bad.
(3) List items in the format (1), (2) and (3).
(3) In rare cases, you may find that you just can't limit yourself to three. Congratulations! This is awesome and that means "Three Things" is working for you - you had a lot to celebrate and you need people to know that this day was extra special. Examples might include something your children did or that you had pizza for dinner. In that case, simply repeat the third entry as needed. Some might accuse you of cheating, but sadly it might just mean they've not read the official rules.
So there you go. Welcome to your new happier life where if you are paying special attention you'll probably start finding it easier and easier to point to three things every day that you liked about today.