Picasa Video Direct Link

You can't, at the moment. Use Picasa on your local computer.

I loved this online feature too, and it's sad that it's gone. However, seeing that the capability appeared only online, and later in the application, I suspect that they removed it from online because it's difficult to merge your face/name assignments from online into your local Picasa installation. It's a lot easier, and more natural, to do the tagging before uploading.

3gp is a common shared between the 3 services you asked for. The only limit you have is on Flickr where the video must be shorter than 90 seconds, so only 1 minute and 30 seconds long for this service.