It’s a very good idea to tap the M3 holes on the aluminum frame before moving on. There’s either some paint, or other cruft shrinking the holes a bit - or the holes themselves are just not tapped well enough. In either case, I broke two screws in the frame on my first build attempt - not applying that much pressure even. Prusa graciously sent me a new frame. After tapping the holes on the new frame the screws went in extremely easy (I could even screw them in by hand the first few turns).
Taps are inexpensive - here’s one on amazon.com https://www.amazon.com/Drill-Warehouse-Adjustable-T-Handle-Thread/dp/B0736T5NM6/
