well i known about this site for some time and i decided to finally join. i am new but not a noob to pikmin. i have found infomation on the entire story for pikmin 3.
i found the whole story of pikmin 3 here.
if you notice it is in japanese so translate it with google chrome.
look to the left and you will notice it reads a little cruddy but the the poor translation. if you ingore that it talks about the order of the days in pikmin 3.
i found the area that says "tower of the sorrowful beast" to be a spolier for the final boss. if you notice it means that it is the strange gold monster seen in this wiki. and in the photos its on a tower. put one and one together and you can see it makes sense. it says even more things like how olimar has an "sos" and have you have to save him. i warn you it has all the spoilers so see it if you want.
it also explains the reason why olimar is there.