How can I put one photo in the top center of two other photos?

Question

I have three photos, a.png, b.png and c.png, I need to combine them into two rows (top and down) so that a.png be in the top center of b.png and c.png i.e. it looks like

   a.png
b.png c.png

I used this command

convert \( a.png \) \( b.png c.png  +append \) -append combine.png  

but this command doesn't center a.png in the middle of first raw instead the combined photo looks like

a.png
b.png c.png

Answer 1

Just add a centering option:

convert \( a.png -gravity center \) \( b.png c.png  +append \) -append combine.png