I think what is most important is not to be caught up so much in what everyone else is doing. If you do that you get caught up chasing someone else's ideas of beauty and simply copying. I would highly suggest thinking long and hard about what you personally want to get out of your design. Look at nature and scenes in nature (not just aquariums). When you see something that is beautiful to you, think about why it looks beautiful to you. What aspects really capture your eye, what aspects lead your eye around, etc.
You don't have to think solely in 3's but there are some parts that you do. Think of the Background, mid ground, and foreground. Think of "bones" these are the parts that stabilize the scene and you build around them.
Some other tips in planting think in odds, meaning plantings in 3's, 5's, and such. Having odds generally makes things more interesting to the eye than even.