Layer 2 Router?

So fairly sure that what I'm after is not called a layer two router.

What I need is a device that bridges three network interfaces but using layer three (IP) packet inspection of the frames to forward frames out a specific interface.

Because the ports are bridged it should be invisible to the devices connected. I imagine it would have to do some kind of MAC address masquerading.

I'm wanting to bridge two of the interfaces with the frames sent out the third interface only if the packet inside the frame is addressed to a particular group of IPs.

At the moment I'm looking at something like the Routerboard RBM33G but it's not clear to me whether RouterOS supports what I'm wanting to do. Otherwise maybe an embedded PC running Linux can do what I want. I'm comfortable configuring Netfilter but if this would require any kind of kernel hacking then I'm not interested...