This has been posted more often and there is one big disadvantage as SME Server lacks proper support for encrypting connections with the third NIC and I guess that it will also not implement MAC filtering and other features to secure your wireless AP. The easiest and safest setup is to attach a wireless capable router to your LAN NIC (they come relatively cheap these days) and configure SME Server to see the LAN provided by this router a s a local network.
By default the router will use NAT. The SME server therefore won't see the individual IP addresses of the clients attached to the router. In that case, there is no need (or point) in declaring the router's LAN addresses as "local". OTOH, you won't be able to distinguish via the SME server's logs which client machines are connecting to which services.
Some router's can be switched from NAT mode to routing mode, and in that case you will need to declare a local network, and you will be able to distinguish different client machines in the SME server's logs.
In both cases you need to be careful with security on the router. FOr instance, allowing unsecured wireless access to the router will create an open mail relay. Don't do that.