user: Question: What is the mean huc example code when the example name's lower snake, there are 6 digits, and less than 3 levels?
Columns: ['Name', 'Level', 'Digits', 'Average size (square miles)', 'Number of HUs (approximate)', 'Example name', 'Example code (HUC)']
Types: ['text', 'real', 'real', 'text', 'real', 'text', 'real']
Sample row: ['Region', 1.0, 2.0, '177,560', 21.0, 'Pacific Northwest', 17.0]
SQL:

assistant: SELECT AVG("Example code (HUC)") FROM "Table" WHERE "Example name" = 'lower snake' AND "Digits" = 6.0 AND "Level" < 3.0;