I wanted to try the latest version of Builder (gnome-builder 3.22) so I decided to install it with Flatpak. Here is a screenshot of GNOME Builder.

To run gnome builder nightly with any of my instructions and use all features you need Flatpak 0.6.13 or later. In case you run GNOME Software 3.23 or later I made a .flatpakref for GNOME Builder Nightly. You can download it and open it with GNOME Software to install Builder (note: you need to have Flatpak installed):
gnome-builder-nightly.flatpakref

If that doesn’t work for you, you can also do it by Flatpak’s commandline:

Installing the flatpakref

1. Install gnome-nightly.flatpakrepo

flatpak --user remote-add gnome-nightly gnome-nightly.flatpakrepo

2. Install org.gnome.Sdk from gnome-nightly using the command underneath

flatpak --user install gnome-nightly org.gnome.Sdk

3. Install the gnome-builder-nightly.flatpakref:

flatpak --user install --from gnome-builder-nightly.flatpakref

Manual approach

1. Install GNOME Nightly SDK keys:

wget https://sdk.gnome.org/nightly/keys/nightly.gpg
flatpak --user remote-add --gpg-import=nightly.gpg gnome-nightly https://sdk.gnome.org/nightly/repo/
See also: http://flatpak.org/runtimes.html

2. Install GNOME Nightly SDK:

flatpak --user install gnome-nightly org.gnome.Sdk

3. Install GNOME Nightly Apps keys:

wget https://sdk.gnome.org/nightly/keys/nightly.gpg
flatpak --user remote-add --gpg-import=nightly.gpg gnome-nightly-apps https://sdk.gnome.org/nightly/repo-apps/
See also: http://flatpak.org/apps.html#nightly-gnome

4. Install org.gnome.Builder

flatpak --user install gnome-nightly-apps org.gnome.Builder

EDIT: Sorry i had forgotten to add --user to all the commands, should be fixed now!
EDIT2: in case you are wondering why the flatpakrepo and flatpakref is hosted on the gnome cloud space it’s because I haven’t gotten in touch with the right people to get official files up in the sdk.gnome.org space where they (imo) belong. stay tuned!