VBoxManage createmedium disk --filename /${path}/D.vdi --size 10240 --variant Standard
VBoxManage createmedium disk --filename /${path}/E.vdi --size 5120 --variant Standard
VBoxManage createmedium disk --filename /${path}/G.vdi --size 5120 --variant Standard
VBoxManage showvminfo ${myVMname} | grep SATA
VBoxManage storageattach ${myVMname} --storagectl "SATA" --port 4 --device 0 --type hdd --medium /${path}/D.vdi
VBoxManage storageattach ${myVMname} --storagectl "SATA" --port 2 --device 0 --type hdd --medium /${path}/E.vdi
VBoxManage storageattach ${myVMname} --storagectl "SATA" --port 3 --device 0 --type hdd --medium /${path}/G.vdi
If you don't have enough ports (default is 4) then...
VBoxManage storagectl ${myVMname} --name "SATA" --portcount 5
Add disks in Windows