Formula fields

Formula fields

I would like to create a logical formula in Zoho CRM, but am not having much luck.  Here's what I'd like to do:

We have 3 custom pick list fields.

The first field has two possible choices

 The second field will be pre-filled (with one of many choices)

The third field will be pre-filled.

We would like a fourth custom field to be filled with either what is in the second field or third field, depending on what is in the first field.   For instance, if the first field is "choice A,"  then the fourth field would be a copy of the second field.

I am assuming that the best way to do this is with formula fields.  If so, please show an example.  If there is a better way, please advise.

Thanks!
Brian