Application forms for applying based on the 3/5 rule are NOT likely to be available much, if at all, before the change itself actually takes effect.
Consider for example, the change to the 4/6 rule provided by Bill C-24 which took effect June 11, 2015. The form, however, was not available even as of that day. Similarly the presence calculator.
There is an important practical reason why IRCC might NOT issue or otherwise make available the new form until after the change actually takes effect: to minimize confusion and use of the wrong form by applicants.
A new version of the application form (but not for use when the presence requirement changes) is likely sooner than that, depending on the prospective timeline for changes such as the 3/5 rule. For example, as of today/tomorrow there will be no intent to continue residing in Canada requirement, so Item 10 in the current form needs to be removed.
The application form is revised from time to time without much fanfare. It is an administrative action. Thus, while there is some possibility that IRCC will continue to use the current form for awhile pending further changes, such as the 3/5 rule, unless the timeline for all changes is short there will probably be some interim versions of the form employed, one with the current item 10 removed for example. (As of today, the application form is a version issued last October).