Even spacing

There's not much to add to the good advice that's already been given. Of course, the alignment command would need to be done more than once. because you've got 3 columns and multiple rows  If you want exact spacing, you may already know to turn on Use Spacing and type in that amount in the Align panel.

I think the easiest way to do this would be to start again -- delete the images and File > Place all of them at once. Press and drag over an area. Without releasing the mouse use the up/down/left/right arrow keys to add/remove rows and columns. Before releasing the mouse, you can control the gutter between images by holding down Command or Control and using the arrow keys. When you release your mouse, the images will be placed into the grid.

After that, while all the images are still selected, you can right click and choose fitting. I usually choose Fill Frame Proportionally. Since the center column in your example is offset in relation to the left and right columns, you'd need to select the images in the center column and hold down Shift to move them up.