can figure it out dice damage step

[Nosta]

what would a 3d4 weapon go if it increase to large

[grarrrg]

what would a 3d4 weapon go if it increase to large

3d4 (3-to-12) is roughly the same as 2d6 (2-to-12)
2d6 increases to 3d6

Also, going by the weapon chart (http://www.d20srd.org/srd/equipment/weapons.htm#weaponSize), d4's tend to become d6's when increasing.
Medium > large
1d4 > 1d6 (dagger)
2d4 > 2d6 (falchion)
2d4 ~ 1d8 (roughly equals)
1d8 > 2d6 (longsword)

I'd say 3d6 is what you want.