My impression is that third person is the most common POV in MG and that multiple main characters is common enough not to be surprising. I do think that keeping it to three POVs is the way to go if one of them disappears early and another is more of a side character. (I remember reading a book with four MCs once and halfway through, the author magically sucked two of them off into limbo; which helped, because there were a lot of characters to keep up with in that particular narrative, but it was also odd to be invested in these characters and then have them drop out midway through.) Three feels doable, though.
Your comment about distinct lines feels right, too. Even in third person, giving each kid a distinct outlook will definitely help the reader keep them straight. And really--it's more than just about keeping them straight. You want your reader to care about them, and for that, the more individual they are, the better.