I prefer 4d6-4, range 0-20, avg. 10, odds of 20 or 0 are just 1 in 1,296.
I actually bought a lot of d6's with 0 instead of 6, so you get 0 - 20 as each die is 0-5.
Cool! I did the sort of the same thing:
Roll 3d20.
Attacking: take worst roll. Made success in battle harder than the 60+% WotC...