Your not paying attention DD....If you get one now......then later get two........you first 31 will be weakened from working with the small 27's and it will drag the two new ones down. They will only be as good as the weakest link. What you want to do will work for awhile but it will not give you the trolling time ( hrs. per day that you want) . Then in the end your first new 31 will not be good enough to mix with two new ones. The general rule for multiple battery systems is.....all the same size and type and all the same age.