How to create a new serial frequency:

Koha is installed with a variety of serial frequencies and you can add more if you wish.

Go to Serials, Manage Frequencies to list all the available frequencies:

Click the button for New frequency.

Enter the Description and Unit required.  

I have selected a Unit of year so the prompt for Issues per unit means Issues per year so in this case I enter 1 (for the 1 issue).

As I have selected a Unit of year, the prompt for Units per issue means years per issue so in this case I enter 3 (for the 3 year frequency).

Enter the Display order for the list of frequencies and Save.