Here is the complete step by step guide on how you can activate sun mobile internet promos such as iLoads with your iPhone 3GS. I also included below the speedtest results for April 2014 in case you want to know how fast sun’s offering is when comes to mobile surfing.
Step 1. Before you can do anything turn ON cellular data under the general settings. Once done, the cellular data network settings will appear. At the APN type minternet.

Step 2. Text <ACTIVATE> then send to 2300 using your sun SIM (free of charge). Wait for the confirmation which you will receive in few hours. In my case I received it within 1 hour.
Step 3. Choose from the available iload promos defending on your surfing preference. Applicable on both prepaid and postpaid subscribers.
Example: i15 send to 247 for 60 minutes free mobile internet
You will receive a message that reads exactly like below

Thats it!!! You can now surf with your iphone 3GS.
Here is the speedtest when I tried sun mobile internet in Tondo, Manila.

Based on my experience it is enough to stay connected on Facebook, search for information online, however, you won’t enjoy watching youtube videos using it.
Hope this help and hope that you enjoy surfing with your sun mobile internet.