I accidentally put 3 bananas instead of 1

Did you add 3 separate entries for 1 banana, or one entry for 3 bananas?

In the first case:
- in the android app, tap and hold the entry and then you'll see the option to delete
- in the app, tap on the pencil at the top of the diary and then select the entries you want to delete and tap on the trash icon
- In the web browser version: tap on the red circle in the right hand column

In the second case:
You can tap/click on the food item in your diary to open it and adjust the serving from 3 to 1