Use repadmin.exe from the Support Tools and add 1 line for each of your domain controllers:
repadmin /syncall DC1.Yourdomain.com dc=Yourdomain,dc=com
repadmin /syncall DC2.Yourdomain.com dc=Yourdomain,dc=com
repadmin /syncall DC3.Yourdomain.com dc=Yourdomain,dc=com