Ender3 Info

Worth checking the bed is level.
A piece of paper should slide between the extruder and bed, but be a little bit gripped by it.

slicer software:

• cura
• Prusa slic3r

random blah:

With the holes, I use 0.2 as a Tolerance - so make the hole 0.2mm larger than the thing.
But side-holes are going to be less circular due to the layers. And you can always cut material away.
I very rarely print at 100% infill, usually 30% or something. It's quicker, and usually fine.
Depends what it is. Also, huge lumps of solid prints can warp as they cool (the top shrinks, and they bow)
(the bottom does not shrink because it's on a heated plate)