If you're trying to grow your hair out, the rule of trimming your hair every 3 months becomes counterproductive. Trims are necessary when factoring in the amount of manipulation but if you're utilizing protective styles the majority of the time, trims aren't required at the 3 month mark. But if you DO wear wash n gos more regularly, you will find that trims will be more frequent, although not necessarily every 3 months. It will be important to keep your ends protected and hydrated and minimize manipulation if you're seeking to retain and maintain your length. This also includes a healthy lifestyle which optimizes growth potential.
If you're noticing weak and damaged ends, it'll be to your benefit to remove those as to not affect the healthier length.