Preliminaries (common and uncommon) in Tibetan Buddhism What are the 3 commom and 3 uncommon preliminaries in Tibetan Buddhism?
There are many Buddhist lineages in Tibet and all of these have variations on what they consider to be preliminary practices or ngondro. So in order to answer your question generally you have to look for preliminary practices that are common to all of the main lineages - Kagyu, Sakya, Nyingma and Gelugpa. 

The 'common' or 'ordinary' or 'outer' practices are generally considered to be the Four Thoughts that Turn the Mind or the Four Reminders. These are more philosophical musings that prepare the mind and motivate the person to continue their practice:


Difficulty of obtaining a free and favorable human life.
Contemplation of impermanence and death.
Contemplation of cause and effect (karma).
Contemplation of endless cycle of suffering (samsara).


A poem by Chögyam Trungpa Rinpoche on the Four Reminders:


  Joyful to have
  such a human birth,
  difficult to find,
  free and well-favored.
  
  But death is real, 
  comes without warning. 
  This body will be a corpse.
  
  Unalterable
  are the laws of karma;
  Cause and effect
  cannot be escaped.
  
  Samsara
  is an ocean of suffering,
  unendurable,
  unbearably intense.


It is in the uncommon or extraordinary or inner practices where there is great variability from tradition to tradition but the following are fairly common:


Taking refuge in the three Jewels - Buddha, Dharma and Sangha.
Developing Bodhicitta.
Vajrasattva practice and mandala offerings
Guru yoga.


These preliminary practices are considered essential before journeying into the intermediate and completion stages of Vajrayana which, traditionally, cannot be done without a qualified teacher.

Recommended reading - A Beginner's Guide to Tibetan Buddhism