Formula Fields (Fields are hidden)

Formula Fields (Fields are hidden)

Hello Everyone

I'm a newbie with Zoho CRM and currently using Zoho enterprise.

I'm currently creating a formula field on my opportunity module, I've created three formula in order to find out the total cost for three separate fields like Material Cost, Trainer Cost, and Venue Cost. 

All these fields have value generated from a formula. Now is the tricky part. I like to the add the total cost of these three fields which data type is the formula (currency) type. While creating the formula, these fields are hidden and not showing up. 

I'm not sure whether I'm using a right method. Please help me in case if you know the right way for doing the calculation.

Many Thanks for your reply in advance.

Regards

Manoj Nair