Have used 3.5 whilst tethered with no problems. Was on the top of a 7 floor building though.

Not sure you can force it. It goes with the fastest available where you are, I think.

NB - if you are in the UK and on O2, expect a nasty text/letter from them if you are tethering. They are watching your usage and will spank you. I got the text and then, when I phoned them to have the data connection re-established (they cut me off, the bastards) they simply said, we know that you have tethered your device. That puts you in breach of contract (having read it, it does). First time offender (cough) got away with nothing more than that, though.

Just a word to the wise.