Not quite, it'd be a bit closer to the following:
CR 1 x2 = CR 2
CR 1 x3 = CR 3
CR 1 x5 = CR 4
But again, note that the scaling is wonky and does not hold true for all levels. At low levels, hordes of weak foes mean something because they can still hurt you, as you approach the teen levels they just bounce off. Also it gets even screwier with mixed CR foes, or synergistic opponents, the CR rules assume they work together, but ALSO, creatures with melee sneak attack often don't live up to their CR without a flanker.
Short version: Best way to gauge it is to run a simulation of the encounter before the game on your own. The amount of difficulty is relative to the CR:
Speedbump encounter. CR < level. Crushed like a bug, no effort expended
Routine encounter. CR = party level+/-1. Theoretically it costs 1/4 of their available resources, but this rarely happens unless you have a large mob of high accuracy mooks.
Tricky encounter. CR = party level +2. They have a genuine risk of losing the fight if luck just went the other way.
EDIT:
3d6 isn't actually a lot of damage. Don't just eyeball it, look at the statistics.
Average damage from 1d12 = 6.5
Average damage from 3d6 = 10.5
Four points, which is significant at level 1(since its the difference between a 2 hit kill and a 1 hit kill), but small in the overall scheme of things. Strength adds +3 damage and accuracy per 4 points. Power Attack adds another easy +3 damage.
Basically, it's his one strong point really. He is only one more hit away from being as squishy as the rogue.
As for the rogue and wizard:
Rogue - In melee, getting into flanking means 1d6+2(str should be 14 or better at least)+1d6 sneak attack, for average 9 damage. If using two weapons and in a flank that goes up to 18(theoretically, but because of lower accuracy it averages lower than that).
Wizard - Has better things to do than deal damage usually. His accuracy is cruddy enough that he might want to use the cantrip splash damage, BUT, it depends a lot on what spells are being used. A wizard is an enabler, he makes other people deal lots of damage.