Try this:
=unique(transpose(split(join(";",A1:A2,B1:B2,C1:C2),";")))
Assuming that your data is in the first two rows of the columns A,B,C
I'm sure that there must be a prettier solution, but that's the only one that came to my mind. Actually, this seems a duplicate to this problem: Stacking multiple columns on to one?
Hope it helps.