Saving SQL Server backup file in multiple parts
Normal backup 

SET STATISTICS IO ON

SET STATISTICS TIME ON

BACKUP DATABASE DBNAME
TO DISK = 'D:\DBNAME.bak'

SET STATISTICS IO OFF

SET STATISTICS TIME OFF


to split the size into 3 use the following code, it will split the size of 3 gb into 1 GB each file
you can add more files or less depending on the size you want 

SET STATISTICS IO ON

SET STATISTICS TIME ON


BACKUP DATABASE DBNAME TO

DISK = 'D:\DBBackups\ReportServer\DBNAME _Split1.bak'

,DISK = 'D:\DBBackups\ReportServer\DBNAME _Split2.bak'

,DISK = 'D:\DBBackups\ReportServer\DBNAME _Split3.bak'


SET STATISTICS IO OFF

SET STATISTICS TIME OFF


for more details 

http://beyondrelational.com/modules/2/blogs/88/posts/10153/sql-server-split-database-full-backup-to-multiple-files.aspx