User3 can access table1 and table2 through view3 if user1 grants permission on table1 to user3 and user2 grants permission on table2 to user3.

If user3 has permission on table1 but not on table2, then user3 cannot use view3, even to access the subset of columns belonging to table1.

User1 or user2 can use view3 if (a) user1 grants permission with grant option on table1 to user3, (b) user2 grants permission with grant option on table2 to user3, and (c) user3 grants permission on view3 to that user.