CiviEvent with Sub-events and Conference Slots - how to....?
Creating Parent Event and Sub Event - using conference Slot


Create an Event and consider that as an parent Event - you need to enable  online registration for parent event 
create another event consider this as a sub-event - In conference slot link the parent event and also create a slot and add to the slot - you don't need online registration for sub-event
repeat the above step to create one more sub-event 


Now you can use event info page from parent event and try to book the event - It should create all the three event against the register contact


Regarding Paid Event

I am not sure about booking an un-paid event - I will give a quick test and will let you know 


Regarding Off-line Registration 

To my knowledge - Conference Slot works can only be used for online registration screen 

I guess this helps!!!