When is CB 3.x due?
When it's done. It hasn't even started. So it won't be anytime soon. CB 3.x development will be done in parallel to CB 2.x and our current plugins. Once done it will release as a beta while plugins are converted to individual Joomla extensions. Once both CB 3.x and plugin conversion to extensions is done we will discontinue CB 2.x development. This will take a long time and won't start until Joomla 4 RC is out. A migration extension will be developed to move CB 2.x to CB 3.x.
When is Joomla 4.x due?
I don't know. Sometime this year for sure though.