Effectively, form 2 and form 3 have switched places.
For Russian, languages having Russian as fallback, Serbian, Ukrainian and Belarusian, there used to be a special case where translators could supply only 2 plural forms. The first one would be used only for the case of 1 and the second for all other numbers. This special case has been merged with the explicit number plural forms. From now on, when {{PLURAL}} is uses two forms, the first form will be used for 1, 21, 31 etc., and the second for all other numbers.
Everything is explained at https://translatewiki.net/wiki/Thread:Portal_talk:Ru/Plural_changes_in_many_languages. Please ask your questions there so everyone can benefit from the discussion.
We have updated current translations as well as we could, and marked all messages as outdated. You can review them here: https://translatewiki.net/w/i.php?title=Special:Translate&group=mediawiki&filter=fuzzy
Many thanks in advance for your help.
Cheers!