In most installations, the “Locale settings” screen shouldn’t appear: the value can usually be determined automatically from the language and country selection. The screen is shown in expert mode, or in cases where the locale can’t be determined (e.g. when I install a system in English but located in France).
The value you choose in the third step is used as the default system locale, which determines the default language and locale settings which are used on the system. The locale setting determines the language used, how numbers, dates, times etc. are formatted, the default paper size, how strings are sorted and compared, and a few other behaviours. If you’re not installing in expert mode, the fact that you were asked to choose a locale is surprising — en_US.UTF-8 is the default when you choose English in the first step and the United States in the second. It isn’t possible to combine any language with any locale; if you try to select English and Germany, you’ll be asked to choose among one of the supported locales because en_DE.UTF-8 isn’t supported by default.
As you surmise, the first two steps also determine the initial values for other selections, including the timezone, download mirror, and keyboard layout.
This is documented in the localechooser README.
You can change the locale once the installation has finished. The system-wide default is stored in /etc/environment, and each user can choose a locale too — typically in the login options (if you’re using a graphical login manager), and/or by setting LANG to the appropriate value in their shell’s startup scripts.
