Auto generate and insert three unique pins

[D Green]

Hi I am new to triggers and need some help.

I have a database which contains some user information.
Each record contains three pins.
Each pin must be unique in it's own column as well as it's own row. In short all pins must be unique.

What I would like to do is auto generate on insert the three unique pins.

To do this I think the best way is with a trigger.

Does anyone have a trigger like this they could share?

I have found triggers which generate random numbers but not that check in three separate columns first and then generate random numbers across thee fields.

Any help is much appreciated.