No rules, but if you are using 3 borders, 1/3, 1/3, 1/3 usually doesn’t look very good unless they contrast really good (e.g., two prints and a solid). Better is usually some ratio of 2-1-2 or 3-1-3. Even 1-2-3 often looks good (note these are ratios, not the actual size). Often having something thin contrasts nicely with wider borders. But as others have said: Decide what looks pleasing to you!