Hi,
QoS and VLAN on a VoIP network architecture are not depending on 3CX or any other IP PBX. If you understand basic principles of QoS and VLANs it should be easy for you to implement this things over your VoIP network architecture.
Anyhow there is a document on how to configure a specific router with QoS for 3CX, maybe you will find some helpful information:
http://www.3cx.com/blog/voip-howto/draytek-firewall-voip/