You need to supply additional info on e.g. OS, shell, that you use. Assuming bash:
- the first expression will try to apply "arithmetic expansion" and assume command to be a variable and expand it (presumedly to zero).
- the second is a "command substitution" and expand to command's stdout in lieu of the command name.
- the third construct will apply comand substitution and then arithmetic expansion, so make sure the command sub. will yield an integer value.
All this can be found in the man page of your shell.