I left my laptop charged and forgot about it for 2-3 hours how bad is that on a scale of lets say 1-10?

Try not to keep it plugged in all the time.
unless you can change the option to stop the laptop from charging the battery when 80/90% capacity is reached.
if not, just charge till about 80 max 90%, use it till around 50% and then charge it back up.
this will give you the most cycles out of the battery.
Lithium based batteries don't like heat, so don't leave the laptop in direct sunlight or the hot car…

if you don't mind getting a hard shutdown when the power goes out, i recommend taking the battery out after its charged/discharged to around 80% capacity and put in the fridge, NOT freezer. Store it for 2-3m and charge it backup to 80%, store it again.

Check what a small UPS from apc would cost and that can provide power for a few minutes to save work and shutdown the laptop. Replacement batteries for those are around 20-30$, much cheaper than buy a laptop battery and the UPS can be used with any brand/computer unlike the laptop battery…

On a scale of 1 - 10? Zero. You did absolutely no damage.

The charge management circuitry built-in to your laptop stopped the charge cycle when the battery was full. You can't overcharge a modern Lithium-Ion battery pack.

Modern devices allow for this and do not overcharge.

It shouldn't cause any problems.

I will go along with Joe here The charge management circuit will reduce the charge if not to zero to minimal
So keeping your battery fitted will have little effect.