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 to t. Check its value with C-h v.
• The faces of the org-headings should have a background configured. Customize them with M-x customize-appropos-faces and search for org-level. It should list 8 org-level faces that you can then customize.

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 to t. Check its value with C-h v.
• The faces of the org-headings should have a background configured. Customize them with M-x customize-appropos-faces and search for org-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.