In SQL Server the foreign key constraints must be dropped before dropping the table.
Please use the below script to find and drop all foreign key constraints of the table.
--Find and drop the constraints
DECLARE @dynamicSQL VARCHAR(MAX)
DECLARE MY_CURSOR CURSOR
LOCAL STATIC READ_ONLY FORWARD_ONLY
FOR
SELECT dynamicSQL = 'ALTER TABLE [' + OBJECT_SCHEMA_NAME(parent_object_id) + '].[' + OBJECT_NAME(parent_object_id) + '] DROP CONSTRAINT [' + name + ']'
FROM sys.foreign_keys
WHERE object_name(referenced_object_id) in ('table1', 'table2', 'table3')
OPEN MY_CURSOR
FETCH NEXT FROM MY_CURSOR INTO @dynamicSQL
WHILE @@FETCH_STATUS = 0
BEGIN
PRINT @dynamicSQL
EXEC (@dynamicSQL)
FETCH NEXT FROM MY_CURSOR INTO @dynamicSQL
END
CLOSE MY_CURSOR
DEALLOCATE MY_CURSOR
-- Drop tables
DROP 'table1'
DROP 'table2'
DROP 'table3'
