Stack images vertically and center

I have several images of different widths and heights that I would like to stack vertically about their respective centers. I've tried

convert 1.jpg 2.jpg 3.jpg -append out.jpg

But this aligns them to the left, I'd like it to be center-aligned.

Incorporate usage of the gravity ImageMagick command line option as in the below examples.

Side by side

convert 1.jpg 2.jpg 3.jpg -gravity center +append out.jpg

convert 1.jpg 2.jpg 3.jpg -gravity center -append out.jpg

Additionally, you might also consider using the resize ImageMagick command line options to scale the pictures up in size to use more of the page.