I know I'm a bit late but I'll leave here my experience in case someone still needs to know if it is possibile.
After doing some tests I figured out it is indeed possibile to connect the Pi3 to two different networks. I configured the dhcpcd.conf file to assign two different addresses (static or dynamic) to wlan0 and eth0 and the Pi could reach hosts from both network and was able to be reached from both networks.
What I didn't solve:
The only "problem" still present at the moment is that, connecting the Pi to two different networks may cause the Pi to stop being able to reach hosts outside those two networks (i.e. an internet host like a web server). Let me explain:
Let's say I connected the Pi to: WLan#1 throught WiFi and to Lan#1 throught Ethernet. If the external gateway (the one which is connected to WAN) is on WLan#1 I won't be able to reach external hosts from the Pi itself since the device automatically uses
eth0 iface as the default one (so it where it redirects all requests to unknown hosts).
This results in the Pi not being able to reach an external network (which is not Wlan#1 or Lan#1, in the example above) unless you manually edit the routing table.
The Pi itself can't know which iface will have the access to a gateway so it is indeed perfectly normal that it uses the Ethernet access as the default one.
The problem is that, as far as I know, there isn't any "simple" way to change the default iface. The only solution I found was to manually add a route entry through shell commands (which isn't ideal in my application scenario).
Keep in mind that the Pi won't become a gateway between the two networks out of the box. If you weren't able to communicate between the two networks you still won't be able to do that, unless you configure some routing inside the Pi (which I didn't look into).