How to Acquire Three Dynamic IP's?

Our office has three completely segregated LANs, each with their own dedicated router. Let's call them office, lab, and VOIP. We are setup this way due to our customer's IT security policies.

I know that it is possible to assign a static IP to each of these three routers, but we really have no need for statics. Dynamics would be fine. Is there any way to configure our Comcast account (or purchase some add-on) that would allow each of our three routers to go out and get it's own public IP via DHCP? So far, depending on my luck, only one or two of my routers can retrieve a dynamic IP (never all three).

We are using an Arris surfboard SB8200 modem in bridge mode (I presume it's bridge mode because our downstream routers are getting public IP's assigned to them).