IMSO 2006 Mathematics - International Short Answer Question 5

A sequence uses only 1, 2 and 3: 1; 2,2; 3,3,3; 1,1,1,1; 2,2,2,2,2; 3,3,3,3,3,3; 1,1,1,1,1,1,1; ... . What is its 100th number?