Whether to Express Positively or Negatively Language of Policy Subject to a Condition

Last month I considered in this post the relative merits of shall not … unless and may … only if, the former negative, the latter positive. I ended up opting for the former, at least provisionally.

Now consider the following:

1. An amendment to this agreement will be effective only if it is in writing and signed by both parties.
2. An amendment to this agreement will not be effective unless it is in writing and signed by both parties.
3. No amendment to this agreement will be effective unless it is in writing and signed by both parties.

The first is positive, the second is negative (with negation achieved by means of the verb), and the third is negative (with negation being achieved by means of the subject).

Which would you opt for, and why?