It depends on how fast you want the privacy. 3 plants will get it sooner, but 2 older plants can easily cover that area. You might plant 3 now, and then later move the center one. I've been amazed at how long the oleanders last, and how they continue blooming so well. I have plants that were planted in 1974 and they are still happily blooming. There is an oleander blight going around the southwest. I don't know if it's in Texas now or not. It is killing a lot of oleanders.
