incomplete data in mpl_toolkits.mplot3d.art3d.Path3DCollection

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After creating a `mpl_toolkits.mplot3d.art3d.Path3DCollection`, e.g.,

```python
from matplotlib import pyplot as plt
from mpl_toolkits.mplot3d.art3d import Path3DCollection
import numpy as np

pos = np.random.rand(3, 6)
print(pos.T)
fig = plt.figure()
ax = fig.add_subplot(projection='3d')
ax.scatter(pos[0], pos[1], pos[2], c=np.arange(pos.shape[1]))
```

I'd like to retrieve `pos` from the figure. As in 2D, I expected `get_offsets` to retrieve the data, but

```python
for c in ax.get_children():
if isinstance(c, Path3DCollection):
print(c.get_offsets())
```

only returns the first two colums of `pos`.

Start by running the reproducer and inspecting mpl_toolkits.mplot3d.art3d.Path3DCollection, especially the get_offsets entry point. Compare the returned data with the original three-dimensional pos array; done means the collection exposes the complete plotted coordinates rather than only the first two columns.

Written by the indexing model from the issue text.