1. I would not use dropbox to attempt to simultaneously access a filemaker file, EVER!
It can corrupt the file, will lead to multiple versions of the file on dropbox etc etc, since the data files are constantly being written to.
Its simply not designed for that purpose.
Instead, turn on Network Sharing in FileMaker Pro, you dont necessarily need server to achieve the same outcome.
A quick read:
Accessing via 3G will require you to forward the network ports (via your router settings) to the PC with filemaker pro on, otherwise you are unlikely to be able to connect to the remote file outside of your local network. If the WIFI connection is the same local network, i.e connecting to the same router, then setup should be straight forward as per the guide above.