The information you provided is rather sparse. It seems like the "Heading 3" line is highlighted because your theme highlights the line the cursor is at?
Here are some pointers:
- What is the value of the variable
org-fontify-whole-heading-line? It should be set tot. Check its value withC-h v. - The faces of the org-headings should have a background configured. Customize them with
M-x customize-appropos-facesand search fororg-level. It should list 8 org-level faces that you can then customize.