I was unable to verify the Ubuntu Server image for a pi 3 from the accompanying sha256sums.txt file on https://ubuntu.com/download/iot/raspberry-pi-2-3.
Output was
sha256sum -c SHA256SUMS.txt ubuntu-18.04.2-preinstalled-server-arm64+raspi3.img.xz | grep ok
sha256sum: ubuntu-18.04.2-preinstalled-server-armhf+raspi2.img.xz: No such file or directory
sha256sum: ubuntu-18.04.2-preinstalled-server-armhf+raspi3.img.xz: No such file or directory
sha256sum: ubuntu-18.04.2-server-amd64.iso: No such file or directory
sha256sum: ubuntu-18.04.2-server-arm64.iso: No such file or directory
sha256sum: ubuntu-18.04.2-server-ppc64el.iso: No such file or directory
sha256sum: ubuntu-18.04.2-server-s390x.iso: No such file or directory
sha256sum: WARNING: 6 listed files could not be read
sha256sum: ubuntu-18.04.2-preinstalled-server-arm64+raspi3.img.xz: no properly formatted SHA256 checksum lines found