Hi!
Couple of questions:
1) Is the WebServer able to ping 192.168.3.1?
2) Is the DB firewall able to ping the ip 192.168.3.1?
3) Is the WebServer doing the routing?
If all is yes, then do this:
Internet Firewall:
nat (inside) 1 0 0
global (outside) 1 interface
DB Firewall:
nat (inside) 1 0 0
global (outside) 1 interface
If all connectivity is working fine this should do the trick, just make sure you have the proper DNS settings on your PCs behind the DB firewall.
Frank