My personal opinion regarding ND gradient filters is that they are a legacy of the film days and are a waste of money now that we can easily combine exposures in post-processing. Blending in post offers greater flexibility (in term of position and transition), avoids putting more glass in front of the sensor, saves set-up time, and means carrying around less equipment. The only benefit I can see with using an ND grad is if you regularly shoot subjects where there is a lot of movement between frames, which would potentially result in ghosting when blending exposures.
Without knowing what you tend to shoot, it's hard to comment on your choice of solid ND. However, in the past I found 3 stops wasn't enough to make a huge difference in shutter speed for things like blurring clouds or shooting waterfalls in sunlight. But if all you want is, for example, to introduce a bit of motion blur in street shooting, then 3 stops might be okay.