[riverkid]

Originally Posted by dihen

To me, the "will not" part sounds ambiguous because the "not" could be modifying either "will" or "have".

No, the 'not' is negating the entire verb phrase, "will have seen" to make it a negative, "will not have seen". I'm not sure why you have a problem with this, Dihen, as the positioning of 'not' is the normal position for English.

And I might make it even worse like this:
"...will for three years not have seen them." or "...will have for three years not seen them.", to prevent "not" from negating the phrase "for three years". Do you still consider them grammatical?

Dihen's new example: "Come tomorrow, I will, for three years, not have seen them."

I expect that the sentence above is what you meant. In that case, of course it's still grammatical, Dihen. Again, as has been pointed out, it's simply not the norm.