If you have a rooted phone you can use your phone as a WiFi hotspot. Your Thrive however, won't connect to the Ad-hoc network that your phone then creates. So you will then need to root your Thrive and ensure that the ad-hoc patch is installed. You will then be able to tether your Thrive to your phone, and will be able to surf at whatever speed 3G can give you. I wouldn't think that 3G would be sufficient for a lot of web stuff, and probably not for streaming but I really don't know.
I just recently, got my Thrive. My phone was already rooted, but it took no time to root my Thrive and get it connected on to my phone's 4G WiFi hotspot. I was getting 12 Mbps+ upload and 6 Mbps download. Not the greatest, but good enough to do what I needed to do.