Hi
From your topology I understand that your default VLAN is the one where your office PCs connect to.
How to configure this is fairly simple: just make sure that the inter-switch (and the router) ports are tagged on all 3 VLANs. This will allow clients on each VLAN to reach the firewall, which will take care of routing. Your firewall must have an interface (IP address) on each VLAN, and this must be configured as default gateway to the clients on each VLAN. Spanning Tree is a loop protection, so since you don't have loops you don't have to configure that.
The question of devices which are not in any VLAN is a bit ... different issue. You have to remember that each VLAN is also a separate subnet. So if these devcies are on a fourth, separate subnet and your router knows how to forward data to that subnet, it shouldn't be a problem.