Ah, I hate these system storage menus: their attempts to be "user friendly" always make them unintelligible! The set of numbers there do not add up to 8 GB, no matter how I try to combine them (the closest I can get is by assuming that the "miscellaneous files" are included in the "used space" but the "cached" data are not).
But if you've used more than 3 GB of user space on an "8 GB" phone, which will have used about half of that for the system, then yeah, that's a problem. In my opinion manufacturers should have stopped selling 8 GB phones several years ago, because anyone who actually uses the phone is likely to hit this problem. And what makes it worse is that when your free space drops below something like 500-750MB (it varies between devices) it will tell you it doesn't have space to update any apps: this is because Android needs to keep some space free for temporary files in order to run stably, but it makes devices with little storage even less usable.
I'm afraid all you can do is clear app caches (social media apps and browsers can build up some large caches), move what apps you can to SD (not all can be moved, pre-installed apps cannot at all, and moving an app actually only moves part of it, but it's something), make sure your media are stored on SD, and remove or disable apps that you don't use.