Do all 3 institutions need to have a doctoral programme accredited so they would enrol students, or do these 3 institutions join forces and design a programme which is accredited somewhere and then the students can apply?

The answer is in between. It is not necessary to have already at proposal stage the programmes officially recognised and the joint or multiple doctorates in place but for joint doctorates, there is a commitment of several doctoral degree awarding institutions, who have this mutually official recognition of the different curricula. So it is necessary that at the end of the projects, all the fellows involved in the joint doctorates, have a joint, multiple or double degree. It’s a very lengthy process, usually underestimated by applicants. There are strong rules about curriculum design in different universities from different countries.