I am trying to find a mini itx montherboard that supports onboard triple monitor. I tried to search on newegg and amazon, but no luck.
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You won't find such a thing. Almost all onboard graphics cards, especially on consumer motherboards, only allow 2 displays even if they have the physical connections for more. Your best chance here is a graphics card, and if you're trying to stuff it in a tiny cases, chances are that's not going to happen.
There are devices that can "split" a VGA signal such that two monitors appear to be one larger monitor, which gives you a better chance, but generally quality is poor, and resolution is limited. I'd say your best hope is a USB VGA dongle, but if you're doing anything 3d, or particularly high resolution, that won't help.
Also, not a shopping guide, yadda yadda.