Is there any time-line when joomla start supporting bootstrap 3?
Migrating from Bootstrap 2.x to 3.x will take a very long time.
I think it's being considered, but doubt very much this will be in Joomla 4.x, therefore I assume it will be introduced in Joomla 5.x.
Just asked one of the PLT members and Bootstrap 3 won't be used. Bootstrap 4 is currently in alpha so by the time Joomla 4 is released, Bootstrap 4 will be stable and possible shipped with Joomla. They're also looking at non-Bootstrap frameworks just in case.
Joomla 4 WILL ship with Bootstrap 4.