It is not the best time now to switch to a gtk3 DE unless you are OK with the default Adwaita theme. Other themes are becoming compatible with the new gtk3 version by and by.
Anyway, you could remove the contents of your /etc/skel folder and install manjaro-budgie-settings package. The create a new user and log in with the new user to the Budgie session. You would get the look of Manjaro Budgie edition like on the ISO (if Arc theme isn't broken at the moment, but then switch to Adwaita.)
To "unite" the DE's you can copy the relevant configs (you need to find out which are relevant) to the old user folder. If it works to log in to the Budgie session with the old user you can delete the new one.