Did the math
GWF using greatsword (2d6=8.33)
S&B using longsword (1d8+2=6.5)
TWF using shortsword (1d6=3.5)
Assuming an ability modifier of +3
GWF
1-4: 8.33+3=11.33
5-10: 11.33×2=22.66
11-19: 11.33×3=33.99
20: 11.33×4= 45.32
S&B
1-4: 6.5+3=9.5
5-10: 9.5×2=19
11-19: 9.5×3=28.5
20: 9.5×4=38
TWF
1-4: (3.5+3)+1=7.5 W/BA: 7.5+(3.5+3)=14
5-8: (6.5+1)×2=15 W/BA: 15+6.5=21.5
9-10: (6.5+2)×2=17 W/BA: 17+6.5=23.5
11-16: (6.5+2)×3=25.5 W/BA: 25.5+6.5=32
17-19: (6.5+3)×3=28.5 W/BA: 28.5+6.5=35
20: (6.5+3)×4=38 W/BA: 38+6.5=44.5
Now let's remember that twf will not be using their BA for their offhand attack every round, and may be utilizing their bonus for ac on occasion. So at 20 using a different BA (6.5) and fighting defensively (3×4=12) that will be 18.5 less damage leaving 26 damage for those rounds.