One option which I have used (I am sure there a couple of others which I am not aware of) is to go to the last row. Between the wall and the seats, there is enough room for one airline seat atleast. So if you do not have competition, you have space for two airline suitcases. The carry on baggage will feet on the overhead racks. And as you have 3 tickets you are allowed 210 Kgs
All the best. Have a pleasant journey