I tried lots of links to increase my LVM(/root) partition size after assembled new disks, only this one works:
https://www.tecmint.com/extend-and-reduce-lvms-in-linux/
As a summary:
It guided me to create partitions on these newly assembled disks(500GB per each) first and then create logical and physical volumes; then extend volume group by adding this partitions and then resize.
$ pvcreate /dev/sdb1 /dev/sdc1 /dev/sdd1
...
$ vgextend ubuntu-vg /dev/sdb1 /dev/sdc1 /dev/sdd1
...
$ lvextend -l +360380 /dev/ubuntu-vg/root
...
$ resize2fs /dev/ubuntu-vg/root
...
$ vgdisplay
--- Volume group ---
VG Name ubuntu-vg
System ID
Format lvm2
Metadata Areas 4
Metadata Sequence No 8
VG Access read/write
VG Status resizable
MAX LV 0
Cur LV 2
Open LV 2
Max PV 0
Cur PV 4
Act PV 4
VG Size <1.83 TiB
PE Size 4.00 MiB
Total PE 479614
Alloc PE / Size 479614 / <1.83 TiB
Free PE / Size 0 / 0
Hope, it may help.