Not sure what species you are considering. Do you mean Butia capitata (Pindo palm). I have never heard of any palm described as "3 trunk palms". Do you mean you want a cluster of three such palms? They are usually grown as solitary specimens and not place that closely together..., unlike say Phoenix roebelleni (Pygmy date palms which are typically planted in cluster of about 3).

Try eBay for seeds..., as long as the seed is fresh, it will germinate.

I also do not quite know what you are referring to by 'three trunk palm'. Where did you get the info? Maybe do a google? I really can't go on that much information (sorry!)