You will only need to worry about 2 of the 3 because luigi will fin done of them after u get him to help u search for power stars(fyi i tis located in battlerock galaxy). The first one is in space junk galaxy where after u feed a hungry luma it will transform into a yoshi head planet and when u destroy all the goombas on it u will get a green star. The other one is in dusty dune galaxy and it comes from a planet created by a hungry luma under neath one of the platforms after u fly off the starting point.