Samsung Galaxy S3 has an amazing feature which is auto-complete. Auto-complete is very helpful in saving time. You can also add custom words to your Samsung Galaxy S3 auto-complete dictionary by following below mentioned steps.
1. First select the Settings app from the Galaxy S3 home screen.
2. Now scroll down and select Language & keyboard.
3. Now tap Personal dictionary>Edit personal dictionary> and then the plus icon.
4. Finally enter the word or phrase you want to add and tap OK.
Note: If above steps do not work for you then you can try with following steps (Recommended by Bo-Ho Kim)
Settings–> language and input–> Samsung keyboard settings–> predictive text (click on the actual list item, not the toggle button)–> my word list
So by following above mentioned steps, you can easily add custom words to Galaxy S3 and your phrase or word will now appear in the auto-complete dictionary.