Always use one slow connection in preference of a "faster" one

The short anwer is NO, you cannot route packets based on metrics when using a "dial up" connection and your HSDPA connection is classified as dial up connection.

Microsoft routes all IP packets via DuN if established and will fence off the local network to prevent external dial up intruders from invading your DSL/Gb LAN.

This legacy design was implemented in the mid 1990's and is useless but it deeply embedded into your networking stack that either 3rd party routerware or Internet Connection Sharing tweaks will be required to circumvent and accomplish what you're trying to do.

A connection profile manager is also a good choice such as IBM/Lenovo's Access Connections Manager which detects all existing connections and uses the fastest, but I've never tried installing it onto anything but a ThinkPad and unaware if it will function on any laptop or would be compatible with your 3G adapter and drivers. Microsoft may offer a connectiopn profile manager but I'm ignorant of one nor would presumptuously claim to be aware of all Microsoft software.

Typically, Windows DialUpNetworking equipped users have accepted this design and consciously disconnected their DuN connection when using a LAN, more than likely you've discovered this too.

Your question begs a follow up as to why is it necessary for you to mount two or more simultaneous IP connections via your LAN and 3G. I hope you don't foolishly believe you can actually etherchannel the connections to increase your bandwidth.

Another reason I'm speculating about your intent is possibly to simultaneously download from a limiting site like Rapidshare that would only allow one file transfer at a time, if that is the case then you should use two different browsers and set IE to use only your 3G dial up connection while another browser like Firefox to use only the LAN and this would enable you to download your big porn / pirated movie files two at a time.