It depends on the level of the charm, which is equivalent to the level of the monster that dropped it. Your character level has nothing to do with the 3 pgem rerolling. Character level ONLY matters with the Crafted Item recipes.
Most people reroll small charms since those tend to have the highest value if you get a good one. Most of the popular mods have an item level under 40 (Fine is lvl 21, +5 resist all is 25, 7% MF is lvl 33, +20 life is lvl 39), so you can use charms found in Early NM.
GC's are re-rolled to hopefully find skill charms, but IMO, it's easier to just find more GC's than the amount of PGems it would take to get the one you want.
3 Perfect Gems (of any type) + 1 Magic Item = 1 New Random Magic Item of the Same Type
You can use this formula to get another chance at a Magic Item of the same type. The ilvl = ilvl. This means to use 100% of the ilvl of the input item. It's important to note the difference between "the ilvl is set to a fixed number" like what happens in some recipes and "the ilvl of the output is exactly the same as the ilvl of the input" as is the case here. So if you're cubing for +skills Grand Charms, for example, you want to use Charms starting from late Nightmare, not ones that dropped in the Normal Blood Moor. You can use this formula (with an Amulet) to create a +2 Skills Amulet. And you cannot Gamble one before Character Level 84. Be warned however, that the chances of getting that on an Amulet might be very low (considering everything is available) and it might take a lot of tries. Rare Items cannot be used with this formula to create new Rare Items. You can also use this on Charms and Jewels