I've done a bit of research, and the N95 is defiantly coming to Three. The time frame isn't set (or at least, isn't announced, and I've been unable to find one...), however it's likely to appear within the next few weeks or months.
UPDATE (31/05/2007): The manager at my local 3 store was told that the phone would be in by the end of this month. It's not actually going to happen, but it does give the impression it will be happening any time now. Perhaps within the next few weeks. Someone discussing this on 3g.co.uk suggests that Three don't want to release another buggy handset, and are holding back on releasing this phone for a while.
UPDATE (01/06/2007): Another place worth watching might be here http://www.x-series.org/2007/05/08/nokia-n95-is-coming-to-three/trackback/
- apparently the first week of june is looking likely!
UPDATE (12/06/2007): Three called me today, and explained it would be a while, they are still testing. They could unfortunately give me no idea as to how long it would be, but it is coming (apparently).