I would stick with the image-specific functions.

import

In[]:=
image1=Import[FileNameJoin[{NotebookDirectory[],"threepages-1.png"}]]
Out[]=
In[]:=
image2=Import[FileNameJoin[{NotebookDirectory[],"threepages-2.png"}]]
Out[]=
In[]:=
image3=Import[FileNameJoin[{NotebookDirectory[],"threepages-3.png"}]]
Out[]=

spacer

There might be a built-in option that would do this more elegantly, but I’ll just create a new image to be the spacer. I’ll make it the same height as the other images.
In[]:=
spacer=ConstantImage[Gray,ReplacePart[ImageDimensions@image1,1->10]]
Out[]=

assemble the images together