Yes you can take these 3 as best subjects if you have scored more in these 3 subjects than the other subjects. But you also need to go through the rules while filling up the form because for few subjects and for few colleges there are some compulsory subjects whose marks are required while filling up the form . For example if you are applying for maths/stat honours your marks in maths will also be required.  So you need to go through the rules and check whether the marks of any subject is compulsory for applying in honours of a subject. Otherwise you can give the marks of these 3 subjects where you scored the most.