I know that John Barrowman has spoken about hopefully doing a S4 of Torchwood, and I think that GDL has mentioned some things to. I don't think your predictions for S3 will be correct-- I don't think it's the last we'll see of Jack in Torchwood, nor do I think Ianto will leave at the end of it-- but I do think they could very well apply to a S4.

ETA: I've seen a lot of people speculating that Jack/Ianto will be over by the end of S3, and I have to wonder, does this merely stem from a sense of narrative inevitability, or does someone know Something From the Powers that Be?