3-way light circuit to an outbuilding with a disconnect

You're allowed to run a switch spur back to the main building

The good news is that you are permitted by NEC 225.30 to run a 3-way switch spur (two travelers and a hot or switched-hot) back to the main building from the outbuilding, although that takes up the single circuit the Code allows you to run back in this situation. I would run it in a separate conduit, though, to avoid hitting derates due to having more than three current-carrying conductors in a single conduit if you decide to pull a feeder through the main conduit to the outbuilding.

As to that disconnecting means

While you could use an ordinary or key-operated lightswitch as the outbuilding disconnect for a single branch circuit, I tend to prefer non-fused pullout disconnects of the type commonly used for air conditioner disconnecting service for this job. They can handle both single-circuit and multi-wire branch circuit situations, are weatherproof (NEMA 3R) by default, and quite inexpensive for what you get. They're also distinctive in that you won't mix them up for a lightswitch when you're working in the shed!

If you really want a diagram...

If you really want a diagram of this all, I edited the diagram from your deleted question to depict what I'm talking about, as shown below. Note that the travelers and switched-hot all pass through the disconnect without being switched by it, since they are powered from an always-hot feed that's already subject to that disconnect. Note also that especially with an AC disconnect, you'll need to be careful to land your equipment ground on the disconnect's bonded, while your feeder neutral does not stop at the disconnect at all; instead, it is simply spliced or pulled through.