Mustrum_Ridcully
Legend
I had a similar thought. "Common" MM Kobolds can stand in for Minions.Yeah, I hear you.
To the OP, I'd say start them at level 3. 4E assumes high starting hp, every class can heal, and minion rules, all things you probably won't bother adding to your 3E rules just to make KotS fit.
Kobold Skirmishers => Kobold Rogue 1-2
Kobold Soldier (Dragonshield) => Kobold Fighter 1-2
Kobold Artillery (Slinger?) => Ranger with Slings
Kobold Controller (Wyrmpriest) => Sorcerer (optionally: Give them "Bless" or a similar "Leader" spell, possibly giving him access to some divine spells, since the Wyrmpriest is a Controller (Leader).)
Similar Conversions can be done for the Goblins and Hobgoblins. You might want add Warrior levels to the Goblins and Hogboblin commons/minions.
I would ensure that the group gets a Wand of Cure Light Wounds early - but that's what we did always in 3E, anyway.
You might want to restrict the starting equipment to normal 1st level gear, though, to ensure that the lower attack values of the NPCs still work, though I am not certain that the 2.700 gp for 3rd level PCs really make such a big difference.