How do I expand the root's volume size?

I have Ubuntu Server 18.04 LTS running off a 16GB SanDisk USB pendrive in my server. From what I can remember, when I installed Ubuntu on there I had enabled LVM support. For some reason, when I ssh into my server, it says / is using 99.6% of 3.87GB, but doing sudo fdisk -l /dev/sdm says:

Disk /dev/sdm: 14.6 GiB, 15664676864 bytes, 30595072 sectorsUnits: sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 512 bytesI/O size (minimum/optimal): 512 bytes / 512 bytesDisklabel type: gptDisk identifier: 7DF91AEG-9DE1-43B2-A7C7-EB564B51FEB2Device       Start      End  Sectors  Size Type/dev/sdm1     2048     4095     2048    1M BIOS boot/dev/sdm2     4096  2101247  2097152    1G Linux filesystem/dev/sdm3  2101248 30593023 28491776 13.6G Linux filesystem

And output from (parted) print with /dev/sdm selected gives me:

Model: SanDisk Cruzer Glide (scsi)Disk /dev/sdm: 15.7GBSector size (logical/physical): 512B/512BPartition Table: gptDisk Flags: Number  Start   End     Size    File system  Name  Flags 1      1049kB  2097kB  1049kB                     bios_grub 2      2097kB  1076MB  1074MB  ext4 3      1076MB  15.7GB  14.6GB

Running sudo df -h gives me:

Filesystem                         Size  Used Avail Use% Mounted onudev                                12G     0   12G   0% /devtmpfs                              2.4G  241M  2.2G  10% /run/dev/mapper/ubuntu--vg-ubuntu--lv  3.9G  3.9G     0 100% /tmpfs                               12G     0   12G   0% /dev/shmtmpfs                              5.0M     0  5.0M   0% /run/locktmpfs                               12G     0   12G   0% /sys/fs/cgroup/dev/loop0                          90M   90M     0 100% /snap/core/6034/dev/loop1                          90M   90M     0 100% /snap/core/6130/dev/sdm2                          976M  155M  755M  17% /boottmpfs                              2.4G     0  2.4G   0% /run/user/1000/dev/loop3                          92M   92M     0 100% /snap/core/6259

I have left out my ZFS volume above, as it's unnecessary to include it.

Running sudo vgs gives me:

  VG        #PV #LV #SN Attr   VSize  VFree  ubuntu-vg   1   1   0 wz--n- 13.58g 9.58g

And, lastly, running sudo lvs gives me:

  LV        VG        Attr       LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert  ubuntu-lv ubuntu-vg -wi-ao---- 4.00g                                                    

/dev/sdm is my root drive by the way. Any insight to this would be helpful. I do have ZFS installed managing other disks, but / is ext3 or 4.

One other thing to note is that I have LVM enabled because if my USB drive were to ever go bad I wanted to be able to restore the data to a new drive, whether it is smaller or larger than 16GB, and utilize the whole disk.