RewriteEngine On

RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_URI} !example.php

RewriteRule ^(services|portfolio|contact)$ $1/ [L,R=301]

RewriteRule ^services/$ /services.php
RewriteRule ^portfolio/$ /portfolio.php
RewriteRule ^contact/$ /contact.php