user: Question: Which Position has a best 3-year period of Petrosian?
Columns: ['Position', 'best year', 'best 2-year period', 'best 3-year period', 'best 5-year period', 'best 10-year period', 'best 15-year period']
Types: ['real', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, 'Fischer', 'Fischer', 'Fischer', 'Fischer; Kasparov', 'Fischer; Capablanca', 'Capablanca']
SQL:

assistant: SELECT "Position" FROM "Table" WHERE "best 3-year period" = 'Petrosian';