Hi Sarah Rose,
The very small 3% for SE Europe probably relates to your deep ancestry from way back in time (it is too small to be reliable). It is very common in those of Jewish ancestry to have ancestry in SE Europe which includes Greece - a lot of Jewish lines travelled the Mediteranean with the ancient Greek empire . I also have a trace from SE Europe (11% on FTDNA).
However, not all the DNA websites agree and I have different interpretations of my results on FTDNA, 23andme, Ancestry and MyHeritage. Some do not give SE Europe at all, whereas others are more specific e.g. if you have a detailed K36 analysis (avialable on Gedmatch for free and on other sites for a small fee).
I suggest you get your maternal direct line (mtdna) tested and also a brother or father's direct male line (ydna) tested with FTDNA. This will give you further clues about ancient origins. This will give you the haplogroups (DNA tribes if you like) of your direct line ancestors, and where they originally came from. There are some very specific Jewish ones, some common but others rare.
FTDNA has a lot of info on its website about origins, and there are plenty of online guides, as well as papers and books on the subject.
Jill Whitehead, Surrey, UK