SQL Server data, logs, backups files in mounted foldersin C:\ I know that is a good practice to have data, logs and backups files of SQL Server in different disk, for example 


D:\ for Data files
E:\ for Logs files
F:\ for Backup files


If I want to use this disks mounted in folders in C:\is safe?
For example C:\MSSQL\Data, C:\MSSQL\Logs, C:\MSSQL\Backups respectively...
Well, you shouldn't encounter any errors from SQL Server, if that is what you are asking about.  SQL Server doesn't care where the files are located.  That being said, there are good reasons for having different file types on different drives.

Performance - Having different files on different drives means that there are different sets of drive heads reading the data.  Your throughput for simultaneous read/write operations goes up.  Especially if you are backing up to the same drive that your data files are on, this will suck.  

Maintenance - What happens when your drive fills up because the part of your script that cleans out old backups has failed and now your drive is full?  If you had separate drives then just the one then your damage would be limited. 

What happens to your backups when your primary drive fails?  Are they being taken off to tape somewhere else?

If this is a production server then I strongly encourage you to separate out to at least separate Data and Log drives and backup to a different machine.