I am searching for open source free API just like Unity or SIO2 which I can use for commercial purpose. Kindly tell me if there is any API for iPhone 3D games.
AFAIK there are two open source game engines:
I don't know much about irrlicht, but there are some dicussions about it here on stackoverflow.com. I think they support iPhone development for quite a while. oolong seems to have a hard learning curve because there is not much documentation and you have to subscribe to a good old mailing list.
If you are an Indie developer with less than 100k $ / year, Bork 3D engine might be worth to look at - you just have to pay $49. There is one drawback in that you don't have support for blender and you should have to check whether collada files are useful, if you don't use a commercial 3D modelling tool.
Just found this in my bookmarks :-) It's a bit out of date but a good place to start investigations: