A couple of points.
The 3Com SuperStack2 Switch 3300's are manageable and support 802.1q and 802.1p. They can only be managed from VLAN 1 as this is where the management interface is, so you must be able to reach VLAN 1 from your management workstation. You need to configure an IP address (plus subnet mask & gateway) via the console port on the rear. If they are stacked with the Matrix ports (and a Matrix module if there are more than 2) then they can all be managed as a single entity, if not each one must be given an IP address and managed separately. Once this is done they support Telnet, a HTTP Interface and SNMP for management.
If you wish to implemenent a separate Voice VLAN - that is 2 VLAN's per access port, the Data VLAN (untagged) and a Voice VLAN (802.1q tagged). Then you must introduce another IP subnet since each VLAN will be it's own broadcast domain and be required to have spearate IP subnets/networks. To route between these 2 subnets you will also need a router or layer-3 switch since the 3300's only support Layer-2 and cannot route - this is needed if you wish to have IP connectivity between the Voice & Data VLAN's.
The 802.1p prioritisation support on the 3300's is not configurable and is also undocumented. I have asked the question before and was told the switches do prioritise packets based on 802.1p CoS values but there is no way of configuring this, but it is done by default. There is also no way of seeing any statistics of this queueing so unless you test things with something like a Smartbits tester you are unlikely to actually see if it works.
I appreciate this is only a relatively simple network but I would consider getting someone to put together a design. I would also consider replacing these switches with something that can give you features such as 802.3af Power Over Ethernet (PoE) to power your Mitel IP Phones, priority queuing that can be configured and monitored as well as Layer-3 QoS (DSCP) trusting & manipulation support.
I have worked with the Mitel equipment on Cisco switches previously and I would not recommend simply using 802.1p CoS to prioritise traffic.
Good luck
Andy