Studio 3 - no longer exists. Like Brekkie says, we decided it was home to the Early Evening News in the early 1990s and was probably taken out in the mid-1990s. I think it was on a floor which ITN no longer owns.
This studio?
http://www.youtube.com/watch?v=D4cMG_pIcvI
Or was that just in the place where the current London Tonight studio is? It could well be a different floor though, the world maps at the back of the newsroom might suggest it's the newsroom for their old World News service
No, it's definitely not Studio 4 - that's on the ground floor. The studio in that clip is higher up the building on a floor no longer owned by ITN. I'm fairly certain S4 was built for News at Ten in November 1992. EEN moved into the studio in that clip in March 1992 and I'm almost certain they didn't share a studio at that point, as the newsrooms look very different to each other. I suspect Studio 3 existed from the time GIR was built (1990), as there's a clip on TV Ark of Trevor McDonald presenting a Gulf War update from the same floor as your clip in early 1991, see here:
http://www.tv-ark.org.uk/mivana/m.php?p=itn8oclockgulfreport1991&spl=1
Therefore, I suspect that programmes like News at Ten, Lunchtime News, World News, Channel 4 News and the 540 News (during the Carol Barnes/Fiona Armstrong era) originally came from studios 1 and 2 from 1990 onwards, with the Weekend News coming from Studio 3 on the atrium. Then, EEN launched in March 1992 and moved to Studio 3. Later, in November 1992, News at Ten moved atrium-side, and this is when I suspect Studio 4 was built lower down the building. Once they decided that the daytime/weekend bulletins were all going to have the same presentation in 1995, they moved News at Ten to one of the basement studios (another member mentioned a few months ago that during the 1995-1999 period NAT and C4 News shared either S1 or S2, with The Time The Place in the other) and moved LTN, EEN and weekends to Studio 4, thus making S3 redundant.
Taking it one step further, Studios 5 and 6 were added for 5 News and C4 News in 1997 and 1999, respectively. Once ITV News moved to Studio 1 in the basement in 1999, that freed up S4 (on the atrium) for the ITN News Channel and later London Tonight. The only change more recently is ITV News moving from S1 to S2 and 5 News moving into S1 earlier this year.