I've never got this notion that its a trilogy. For me CR and QoS bookend each other nicely, telling the begining and the end of the story. It isn't about Quantum, it's merely about Bond becoming Bond. The next film might or might not feature Quantum but that doesn't make it a trilogy.
It's not a trilogy - apparently they did shoot a different ending, with Mr White again - which would have made it a trilogy by leaving a direct cliffhanger, but they wisely decided against that.
Craig has said that as far as he's concerned he wants and expects the next one to be a bit lighter - not full-on Moore-era camp, but a solid standalone piece of entertainment, as Bond has got past the personal issues of the first two movies and is now properly 007.