Hmm. How do you fit a Ubuntu installation into 3 GB? Are you really referring to RAW format (i. e. a full-blown filesystem image), or is it a qcow2 image of (much) greater size?
root@one-1:~# ls -lh /var/lib/one/datastores/1/f937300663adab8fc9baa4286c6363fa
-rw-r--r-- 1 oneadmin oneadmin 3.0G Feb 19 01:06 /var/lib/one/datastores/1/f937300663adab8fc9baa4286c6363fa
root@one-1:~# file /var/lib/one/datastores/1/f937300663adab8fc9baa4286c6363fa
/var/lib/one/datastores/1/f937300663adab8fc9baa4286c6363fa: QEMU QCOW Image (v3), 21474836480 bytes
root@one-1:~# qemu-img info /var/lib/one/datastores/1/f937300663adab8fc9baa4286c6363fa
image: /var/lib/one/datastores/1/f937300663adab8fc9baa4286c6363fa
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 2.9G
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: false
refcount bits: 16
corrupt: false
In case it’s the latter, and the final size would be >59 GB, then you hit the same obstacle as me.