Peak Design Everyday Sling 3L

Q: Can this fit a canon 5d mkiii with a 24-70 lens attached?

I’m in Nikon guy, just coming back from a trip it fit my D500 and an 18-200 zoom perfectly, with a shade. The pit was best if the lens and body were separated. When I was shooting actively I really didn’t need the case.

Q: Can I fit a Nintendo switch + accessories inside a 3L?

Yes, but I suggest to remove the Joystick.

Q: Would this fit a Canon 70D with more than one lens? Likely going to carry a 10-18mm and 70-200mm in this bag.

Unlikely. I can carry comfortably only two medium lenses with my Olympus E-M1II, and these are definitely more compact than your Canon equipment, esp. the 70-200. Even if you do manage to squeeze everything tightly into your bag, I wouldn't recommend it, as you would compromise both usability (hard to get things in and out, opening/closing zippers, etc.) and durability (less protection for equipment, more stress on bag seams and zippers). Also, keep in mind that there is very little room for any additional stuff like keys , wallet, or sunglasses. The 3L is a great bag for a compact minimalist kit. For more gear, I recommend going for the roomier 6L, or something else like the Turnstyle or Photocross slings (I got the Photocross 10 and it's AWESOME).