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.
UPDATE: When a heading is collapsed, it indeed doesn't fill in the full width of the line.
The background is only filled in up to the three dots (known as the org-ellipsis).
As far as I know, this is normal behaviour and can't be altered.
You'll notice that when a tag is added to a headline (which can be done with C-c C-c), collapsed headlines will be colored including the tag, which, depending on your settings, will be right aligned.