A: At one go - the advantage is that you will still have the basic information of the Foundation level fresh in your mind when you do the practitioner level course and exams. Disadvantage is that, if your situation is such that you are unable to get away from work for more than the three days of the foundation level course, or you need a lot more time to assimilate / memorize the facts, you will be under pressure to do a lot of extra hours of home work. However, see pre-course.
In some certification courses, you will be sent materials and directions for doing some studying at home, before attending the course. This is because of the intensive nature of the course, where a lot of information has to be absorbed in a short period of time. This is to ensure that you have the maximum chances to pass the certification exams. Although some delegates can get away with not having done the pre-course studies, they have had to work extra hard in the evenings to do their home-work. This is especially relevant if you choose, like most delegates, to do the Foundation level and Practitioner at one continuous go. (for choices between doing the two levels at one go or separately...)
This is not going to happen. PRINCE2 does not stand for PRINCE version no. 3. Hence there will not be any PRINCE3 as far as we can see. PRINCE was originally a project management method for IT (EDP in those days) systems projects. It stood alongside SSADM (Structured Systems Analysis & Design Method). PRINCE became very popular even with non IT projects, largely because it was royalty free and popular. Of course, the method had to be individually adjusted to each type of project. This led to a move to redesign PRINCE to make it generic, so that it can be used for any type of project. Hence PRINCE2.