If you used the tutorial thread Mounting network shares with autofs (alternative to fstab) then you can specifiy the parm -vers=3 in the auto.nfs.shares file like
# cat /etc/auto.nfs.shares
/mnt/mytest synologyds214se.fritz.box:/volumeUSB2/usbshare
/mnt/music -vers=3 synologyds214se.fritz.box:/volume1/music