I installed redhat, suse and fedora then after that I can't log in.
I installed linux redhat and linux suse and linux fedora then after that I can't log in because it tells me in all operation the password fails.
How can I fix a failed password in all operating systems?
If you are able to log in as 'root' user, then log into the shell (tty) and run the following, it will ask you to reset the password for the mentioned user:
# passwd user
(replace the 'user' with your actual user).
If you are not able to log into root, then you need to boot into single user mode.
Not that I don't want to help, but this is going a little bit towards 'hacking' the 'root' user account. So, excuse me.
Google is at your disposal for further queries :)