SP2013 - Workflow - Multiple users need to modify same form with read-only parts

Question

I have a list with 9 fields. I need to create a workflow that looks like this:

1. User1 insert a new item in the list BUT can only write in the first 3 fields.
2. Workflow starts (automatic start)
   
3. User2 gets an email, approves the first 3 fields, can't modify them but can write the fields 4,5 and 6.
   
4. User3 gets an email, approves the first 6 fields, can't modify them but can write the fields 7,8 and 9.
   
5. Workflow ends.
   

This should be made with a simple form like:
Step 1 -> User see all the form blank and field 4-9 greyed out
Step 3 -> User see all the form, fields 1-3 compiled and fields 7,8 and 9 greyed out
Step 4 -> User see all the form, fields 1-6 greyed out and 7,8,9 to be compiled.

I'm using SP 2013, I have Designer 2013 and Infopath 2013. But I really don't know how to make this thing work.

Answer 1

There is no Field Level Security in SharePoint, only Item Level Security.

To make this work, you can create 3 Forms (Lists) that user(groups) can change. Each Form has 2 ListViews on it displaying the other entries. And some Workflows to glue it all together.

You can choose to continue with these 3 Lists or make you Workflow more intelligent and maintain (with elevated permissions) one Shadow Form.