I might be being stupid, but our fuel is also priced in 3 decimals like £1.149 per litre, even though "point nine" is an non existent price, so I'm assuming it's rounded off on fuelly, like it is at the pumps.
Where I live in Canada, fuel purchase receipts show 3 decimals of accuracy for Liters. That's what I've always entered into Fuelly, and that's what has always shown on Fuelly's web site.
When I purchase fuel in US gallons, I convert to liters, and put in the full 8 digit precision into fuelly, and it rounds off to 3 decimal places. I don't ever recall seeing only decimals for the Liter values.
The price per liter I see, in Canadian dollars, is also accurate to 3 decimal places, such as $0.899. All the math Fuelly does with respect to multiplying price per liter by liters purchased works out to the dollar value on my receipt.
2015 Audi Q5 "Progressiv" + S-Line + Scuba Blue, 3.0L V6 TDI
(Highest fuel economy for all Audi Q5s on Fuelly!)
The pumps show to two decimals, and you put up to two decimals in fuelly, but scroll down after adding a fuel up, and there's 3, I think that's why the op is confused. I've noticed when filling up, when I used to round off my purchase, because of the silly "point zero nine" price, you could squeeze a tiny bit more fuel in, but the price wouldn't move. I'm too stupid to do the math, but fuelly must be rounding up, or down the quantity to three decimals somehow.
It's working like any other spreadsheet. You can enter any number of decimals you want, but the display will only show the two or three it is set too. However, any calculations will be with the entered value.