Hi and welcome. When you say "three litres a night" - are you drinking it or peeing it? Thirst and frequent peeing are well known as common diabetic symptoms, although not everyone gets them - I got the thirst but not the peeing, for example.
If you think you might have diabetes, the usual method of diagnosis these days in the UK is a blood test called an HbA1c, which your GP would normally arrange for you. Normal blood glucose is under 42mmol/mol, with most people around 38: and you would "automatically" be diagnosed as Type 2 diabetic, symptoms or no symptoms, once your blood glucose level reached 48. The issue with having high blood glucose is that elevated glucose levels over time damages nerves and capillaries, which leads to other problems best avoided.
It's fairly common to have a range of diabetic symptoms long before the 48 level is reached though (I did), so you can't regard a reading of under 48 as "safe". I was probably diabetic from around 2009, which is where the first symptoms started, but despite them getting worse and increasing in number my practice told me firmly that I wasn't diabetic - until the end of 2019 when they finally told me I was.
The good news is that there is a lot you can do just with changes in diet to control and turn round the condition fairly quickly, and if needed there are a number of medications that can help. I got almost all of my information from this forum, and haven't had a non-normal blood glucose HbA1c reading since January 2020. Symptoms gone, feel great, and subsequently lost around six stone.