First off, kudos for coming up with a way to land a sneak attack, get away, and make a spell attack in the same turn using Fancy Footwork/Quicken. This is just the sort of ability "engineering" that keeps me up all night thinking about multiclass combos. It's a cool idea but I don't know if it's cool enough to give up 3 levels in either class to pull off. There is some synergy with the ability stats, but that is true for bard, warlock and paladin as well. You didn't elaborate on your character concept, so I don't know whether you envision a sorcerer who takes levels of Swashbuckler for Fancy Footwork and Rakish Audacity or a Swashbuckler who dips a few levels of sorcerer for the occasional SA - Quickened Firebolt combo. Either way you are delaying some fairly strong power upgrades in either class.
In terms of pure damage output, a 6th level sorcerer will completely outclass a sorcerer 3/swashbuckler 3. Having said that, I think there are some cool things you could do as a Swashbuckler with just your three levels of Sorcerer. Since your bonus action can happen before or after your action, you can work all sorts of shenanigans with 2nd level spells. After your SA, cast Spider Climb and run up a wall and out of reach. Cast Invisibility to give yourself advantage on your SA. Or make your SA and then turn Invisible. Before or after your SA, cast Misty Step or Mirror Image. And best of all, cast Hold Person and if it sticks you can SA with advantage and auto-crit. (I think I've found a new NPC villain to give players a headache!)
Which class would you pick to start? If you start rogue you lose the sorcerer's Con save proficiency, which you might not care about if you're only dipping sorcerer. If you start sorcerer, multi-class into rogue doesn't give you any weapon proficiencies, so the only sorcerer weapon you could use for a melee SA is a dagger. SA requires a melee weapon with the Finesse property; a quarterstaff is Versatile, not Finesse. You could still SA at range with a crossbow.
Also, Cunning Action competes with both sorcery point/spell slot conversion and Quickened Spell for your bonus action. You only get one bonus action per turn, so you won't be able to Bonus Action Dash, for example, and then Quicken a spell.
If you like the idea of slipping away after an attack to cast a spell, the sorcerer can already do this on their own without needing 3 levels of rogue. A 3rd level sorcerer who has an enemy next to them can Quicken a Shocking Grasp, and if it hits make a Booming Blade attack and then move away without triggering an AoO, or move away and cast another cantrip, or just use Disengage if Shocking Grasp doesn't hit. If you like the bonus to initiative that you get from Swashbuckler's Fancy Footwork, two levels of bard will get you Jack of All Trades which adds half your proficiency bonus to initiative. The initiative bonus is less than Fancy Footwork, but so is the multiclass cost and with bard you keep up with your spell slot progression just the same as if you advanced in sorcerer.