Pretty sure the sugar is necessary for preserving. You could make it without sugar, but it would just be pulpy juice-water (the sugar is also a thickening agent), and you would have to eat/drink it pretty fast because it will go bad quickly (I would guess a couple days)

When I was a little kid we used to make blackberry jam. You can just pick wild blackberries in some places, which is quite a lot of work, but hey, you're out in nature, it's fun, and it's free. Looking back I think it was mostly my parents picking berries while my sisters and I were running around and playing in the forest and eating half the berries our parents picked. The recipe for making jam is indeed just berries, water and sugar. We used a large pot though, not a frying pan. Just cook and steer until it's done. Pour the jam into a jar while it's still hot, and screw the lit on. As the jam cools it'll create a slight underpressure in the jar, helping preserve the jam and tightening the lit even further. Sealed properly it can stay good for a long time. One year we kind of overdid things (my sisters and I were a bit older, and actually starting helping instead of 'helping') and ended up with over 300 jars of blackberry jam. They were still good 10 years later. Self-made jam tastes much better than store-bought jam. Whether that is because it actually tastes better, or because your brain just thinks it tastes better because you made it yourself, I don't know. But it doesn't matter, the end result is the same.