Paulownia (sometimes called kiri in Japanese or princess-tree mostly by shyster Paulownia salespeople) is a weed that can be fairly invasive (environmentally destructive) in many locations. There is a paulownia tree industry out there that spends a lot of effort on promoting this piece of botanical trash, mostly by bending or covering up the truth. It should not be hard at all to germinate the seeds, but I definitely wouldn't waste my time.

If you have fresh or properly stored seeds of the other two trees, germination should be fairly easy. Keeping the Franklinia alive for many years, may be an issue (they are notoriously difficult to grow, but not impossible).