Spsilos01,
My name is Ismael. I'am with Small Support Center. From what iam understanding you want to configure 3 ports to pass Vlan 2 traffic and then allow for Firewall to pass DHCP to your Vlan 1 and Vlan 2. If your three ports are not going to have an ip phone to them then it would be safe to untag your vlan 2. The uplink from SG300-Firewall would be trunked and then you would untag your management vlan 1 and tag vlan 2. This should allow both vlans to get DHCP addresses from your firewall. Just make your firewall vlan aware. Let me know if you need more on this.
This link has screenshots of configurations for all types of small business devices: It could show you step by step on how to configure vlans, tag and untag them.
http://sbkb.cisco.com/CiscoSB/Loginr.aspx?pid=2&login=1