Thank you for your reply, Now I understand better how it works.
I thought about keeping on doing the family three with the FTB, and upload it.
If some one in the family does the same, download the FTB, make there three, upload it to there site, and then search for matches, will there have my matches, and will I get theirs?
If the trees have similar people and are on different sites but also under different accounts (site created by different person with different email address), then you should get matches between the trees.
