A malicious user can cause frames to hop from one VLAN to another depending on the configuration of the switch. This isn't a bug, but rather the result of the dot1q standard and all vendors have this "problem".
As mentioned, don't use native vlans to carry any traffic, and don't provide any trunked ports to any devices. This will mitigate the concerns with VLANs themselves.
The other problem is that a malicious user in the DMZ or on the Outside can hack at the switch itself to gain admin access. If this happens due to poor security configuration of the switch as a host, the attacker could simply make the port of another device on the switch that was formely in the DMZ/Outside in the inside VLAN giving full access.
The last problem is human errors in configuration have a high risk on creating new problems. With three switches, this isn't a concern as each network segment is both logically and physically separate.
Cisco even has 8 port switches now that are cheap. It's hard to make a arguement these days for using one switch for all three purposes.
