Adding text automatically using styles (or GREP)...?

I have to create a book with hundreds of similar er... (what's the English word) short lists. I have attached three jpg's.

Number 1 shows how the text is delivered to me (showing only 4 'lists' as an example). 

Number 2 shows what it looks like after I applied the styles I made for each line (there are 5 different styles): I use the option to automatically assign the next style so everything has the proper format/lay-out as desired with just one click, so including the extra spaces and the line. (Excuse my bad English... 😉 ). 

So far, so good.

Now the problem is number 3: the last three lines of each 'item' needs to get a 'header' in a different font. I wonder if it is possible to automatically add 'Header 1', 'Header 2' and 'Header 3' in these specific spots including a tab behind it AND in that specific bold font.

Can this be done somehow using the styles...? Or GREP perhaps? Or in any other way? I'd hate having to add those three headers manually everywhere.

(I do know how to give 'Header 1' etc. a specific substyle using GREP but then I'd still have to add the words 'Header 1' etc. manually everywhere...)