How to Install Multiple Snaps Simultaneously?

The second example you give works, on the proviso that all three use the same confinement model. If one of them requires the --classic confinement model, then it will need a separate install step.

So simply:-

snap install notes shout hangups

Will work. As you can see here:-

alan@hal:~$ snap install notes shout hangups
shout 0.53.0 from 'snapcrafters' installed
hangups 0.4.4 from 'tomdryer' installed
notes 0.9.0~gitb6e3b34 from 'notes-developers' installed

Parallel installs

snap now also has an experimental parallel install feature documented at: https://snapcraft.io/docs/parallel-installs

As of snap 2.50.1 Ubuntu 21.04, if you do:

sudo snap install hello-world hello-world_foo

it fails with:

error: cannot install "hello-world", "hello-world_foo": experimental feature disabled - test it by
       setting 'experimental.parallel-instances' to true

and after:

sudo snap set system experimental.parallel-instances=true

the installation works.

However, if you run again:

sudo snap install hello-world hello-world_foo

it now fails with:

error: cannot install "hello-world", "hello-world_foo": no install/refresh information results from
       the store

and exits with status 1 which is annoying because I with it were idempotent by default while developing and re-running my setup scripts. This has been asked at: Problem when trying to install multiple apps with snap