Is there any advantage (or perhaps I should ask, *what are the advantages*) of having a separate physical server as a firewall, another for a dmz/internet server, and another as an internal/intranet server?

	Big Bad Internet
	|
dmz/internet server	--------Firewall--------	wireless router
	|
	intranet server, other stuff

For the sake of argument, let's say that the following is true:
-firewall is running a linux firewall (astaro, endian, engarde, take your pick), has a quad port ethernet card connecting as above
-the dmz/internet is runnning selinux (engarde)
-the wireless router is untrusted and ip's are tied to known mac addresses
-on the internal side, the intranet server and asterix/trixbox servers are running selinux (engarde), there is the usual bevy of stuff on the internal side (storage area network, users, etc)
-there are (obviously) switches between the firewall and dmz, and between the firewall and the internal stuff

+Is this overcomplex? Should we instead integrate the (untrusted) dmz/internet and (trusted) intranet, asterix, etc onto the firewall?
+what am i missing from a design standpoint?

Yes I agree you should not host anything other than a firewall on a single host.  There  have been times I have encountered a box with too many things running on it.  If I could find a weakness and gain access, well everything else fell apart as far as security goes.