It used to be three ports, but nowadays it is two ports for remote access.
Here are some instructions for Single Port Forwarding: http://www.linksys.com/fi/support-article?articleNum=140223
What you need to do is follow the steps in the article twice.
1. Follow steps 1-3.
2. On step 4, enter something like "Tablo1" for the app name, 21091 for the external port, 8887 for the internal port, and the Tablo IP address (you should use DHCP reservation to make it stay the same IP forever). Use TCP only. Save.
3. When you repeat everything, use "Tablo2" for the app name, 2109C for the external port, 80 for the internal port, and use the same IP address. Use TCP only. Save.
4. Go back to the Tablo and retest Port Forwarding to see if it works.