mysqldump should be just fine. Since you have only 3GB of data, I expect the mysqldump to be decently fast (1-2 min as a single output). However, since your database is comprised of 3 MyISAM tables expect all three to be locked during the backup. Please keep mind that since tables are locked one at a time, you will not have point-in-time consistency across all tables in the database.
I have some scripts I wrote and still use to perform
- parallel backups of separate databases
- parallel backups of separate tables
If you are concerned with consistent data, you may need to implement something from Option 4 of the link I provided which executes
"FLUSH TABLES WITH READ LOCK; SELECT SLEEP(86400);" in a separate session to lock all writes to all tables, perform the mysqldump(s), and then kill that session holding the tables locks.
If you are afraid to try executing this, you should look into using MySQL Replication. A slave server with the same data can be used to perform mysqldumps without imposing anything disk I/O or overall server load on the master database. You basically do these things on the DB Slave:
Give it a Try !!!