If you would like to display only three related products on the product view page, you can follow these steps:
- Login to the Magento Admin Panel
- Navigate to System > Configuration
- In the Configuration panel on the left, under Catalog tab, choose Catalog
- Expand the Number of Products in Grid After Sidebar to 3 under the Recently Viewed, Also Purchased, and Related Products section
- Save Config
- Flush Magento Cache
Alternatively, if you are comfortable working with code, you can edit the related.phtml template file located at app/design/frontend/[your_theme]/template/catalog/product/list/related.phtml. Find this line:
$collectionSize = $productCollection->count()
and change it to
$_collectionSize = 3
Hope this helps!