dump db3 to a file
$ mysqldump -h localhost -u us2049 -p secret db3 > backup.db3


restore to db named restored
$ mysql -h localhost -u us2049 -p secret restored < backup.db3


Note that you can back up all the databases on the system (assuming you have the credentials to read them) by replacing the database name in the call to mysqldump with the special keyword --all-databases. Here's an example:
$ mysqldump -h localhost -u us2049 -p secret --all-databases > backup.all
Step 3: Create a recurring event to perform the backup on a regular basis

It's unusual for the data in a database to remain static, especially in a Web environment which typically has multiple processes writing to a single database simultaneously. To ensure that your backups are always fresh, it's a good idea to add the mysqldump command to your cron table. Here's an example cron entry that backs up the db3 database every night at 1:00 AM:
0 1 * * *   /usr/local/mysql/bin/mysqldump -h localhost -u us2049 db3 > /disk2/backup.db3