LXD 3.x availability for Ubuntu 16.04?

In LTS releases stability is champion. If you report a bug and the security and bug fixes are significant then the package will be updated sooner rather than later.

OK. Perhaps I need to paraphrase my question....
Should we expect LXD-3.x to be included in next planed release of Ubuntu 16.04 LTS (Ubuntu 16.04.5 - 2nd of August 2018)?

It is _not_ the purpose of a point release of an Ubuntu version to provide updates like the one that you are looking for (lxd 2.0 -> 3.0).

If you want LXD 3.0 in Ubuntu 16.04, there are two options.

1. You can install LXD 3.0.x on Ubuntu 16.04 using the "xenial-backports" repository. More on that at
https://help.ubuntu.com/community/UbuntuBackports
By doing so, if you run "apt show lxd" will show you the 3.0.x version so you can upgrade to it.

2. You can install the snap package of LXD on Ubuntu 16.04. The snap package of LXD has "channels", which are separate versions of the software. There is a channel for LXD 2.0.x, LXD 3.0.x and of course LXD 3.13 (or whichever is the newer version).
The LXD 3.0.x channel is supported until 2018+5=2023.
To install it, run

sudo snap install --channel=3.0/stable lxd

More on channel for snap packages, https://docs.snapcraft.io/channels

Overall, I suggest to use the snap package of LXD (either latest version of LXD, or the 3.0.x channel).
The reason being that the "backports" repository requires some special care so that you only enable the new LXD but not all other updated packages that are available as well.