You need to use combine_options
with MiniMagick to roll all three of your commands together before you write it:
require 'mini_magick'
image = MiniMagick::Image.open('src.png')
image.combine_options do |c|
c.thumbnail '200x200>'
c.gravity 'center'
c.extent '200x200'
end
image.write 'dst.png'
More info on the GitHub docs