Eruvin 54a says that it was taught by Rabbi Eliezer ben yaakov that wherever it says Netzach, Selah, or Vaed it means it will remain so forever. "כל מקום שנאמר 'נצח' 'סלה' 'ועד' – אין לו הפסק עולמית" What is the difference between these 3 words?
An Explanation of Synonyms in Tana"ch by Rabbi Sh'lomo Aharon Wertheimer (pp. 240-241) differentiates the 3 terms in the following way:
Netzach - continuous existence without change
Sela - continual periodic action, as in day after day or generation after generation (cf. T'hilim 84:5 "אַשְׁרֵי יוֹשְׁבֵי בֵיתֶךָ עוֹד יְהַלְלוּךָ סֶּלָה")
Va'ed - unlimited persistence (qualifying the phrase "בלתי בעל תכלית" from Sefer Ha'ikarim with the adverb "בהתמדה")
It says in the introduction to the section that they basically mean the same thing, and I for one find the differences very hard to pick apart even with the prooftexts here and there. The unchanging aspect of netzach appears to be its main differentiator. I think I can imagine the third one meaning having the capacity to go on infinitely, while the second one is more like having infinite confidence that for any given instantiation of the continued thing there was an instance immediately before it and will be one immediately after it.
It contrasts the first two by example of Rav Sa'adya Ga'on's list of mitzvos in which he refers to the mitzva of tzitzis as "netzach" because the Torah commands it "for their generations", and the mitzva of m'zuza as "sela" because it is "day after day and also at night". This sounds like the opposite of the definitions above, but no further explanation is given.
In the same section, it also lists two other synonyms:
Ad - stretching to remote points in time either backwards or forwards
L'olam - forever (perhaps meaning only into the future?)