When using the quick budget option 'Average of last 3 months' it is only calculating the average for the visible previous months on the screen.
Eg. Open your budget the current month is displayed, select any category and use the quick budget 'Average of last 3 month', as no previous months are displayed on the screen it just returns null. If you then make the previous month visible it calculates it based on the one month divided by 3 and so on until all months are visible on the screen.
The same problem occurs when using the quick budget for the entire month.
I don't know how I missed this :P
Here's the trello card:
https://trello.com/c/7xIEsiTe/273-quick-budget-last-3-months-not-calculating-correctly
Thanks :)