Sneak attack damage is suppose to add an extra 1d6 every other level. So 2d6 at level 3, 3d6 at level 5, etc. At least when I played EA it wasn't increasing at level 3. This is a rogue's way of staying roughly on par with other melee classes who get extra attacks at 5th level (and sometimes beyond) where the rogue does not get extra attacks.
It does increase. The calculation is just weird. At level 3 the calculation is [1d6]*2 sneak attack damage instead of 2d6. This results in much larger variance in sneak attack damage.
