Extreme under extrusion Ender 3 Pro
I had a similar problem to you with my Ender-3 but now it is fine.
There are so many causes of under-extrusion that I doubt I can tell you what is specifically wrong in your case but perhaps I can give you some pointers.
In addition to what you have done, check if you can manually extrude the filament by releasing the grip on the filament (depress the lever on the feeder mechanism) and push the filament through with your hand (while the nozzle is heated to the correct temperature for your filament).
If that works well without too much force then it suggests one of two things:

an issue with the feeder mechanism (e.g. worn or clogged up teeth on cogs, spring too tight or too weak) or
extrusion settings on your slicer need modification (e.g. your slicer made G-code with too frequent retractions and this causes the filament to wear down thin preventing the extruder feeder cogs from gripping it. A solution is to reduce the maximum number of retractions possible by altering your slicer settings - this was an important factor for getting my machine to work again).
I found the following YouTube video helpful in giving me pointer of where to start - perhaps it will help you too: