Hi Ruth,
You can merge two GEDCOM files together with Family Tree Builder:
http://www.myheritage.com/help/2008/10/is-it-possible-to-merge-two-gedcom-files-together/en/
Keep in mind that you will have to remove the duplications manually, and then define the ties between the 3 trees using the attach feature.
Regards,
Noam \ MyHeritage