Hello @Arjun6,
As you have all three rooms on the one listing you can't block individual rooms. Your calendar is for your entire listing so if you block a date on it, as you found, no rooms will be available.
If you want to manage rooms individually you have to have a separate full listing for each room. Each room will then have its own calendar which will be independent of the calendars for the other rooms.
The article below explains it.
How do I list multiple rooms?
Steve.