Ideally, you would want the mixture uniform in texture and consistency, so a need to blend all the stuff together.

Not sure how large a berm you envision, but the link will provide rough calculations to follow to determine the amount of soil mix needed. Typically it is going to be something in cubic yards, not cubic feet, like most bagged soil products.

Here is a link that might be useful: berm soil calculations

Personally, I would not want a berm made of pea gravel, sand and mulch. Sometimes, even if if things work doesn't necessarily mean that they are good ideas. Please post a picture of the area where you are proposing to put this berm.