[Unit]
Description=Mount nas3 at boot
After=systemd-networkd-wait-online.service
Requires=systemd-networkd-wait-online.service

[Mount]
What=192.168.3.51:/srv/nfs/nas3 
Where=/mnt/nas3
Options=vers=4,nofail,auto,retrans=5,soft,bg
Type=nfs
TimeoutSec=30
ForceUnmount=true

[Install]
WantedBy=multi-user.target