Hi Wavedave - thanks for the support with the board quiver!! :-) did you try your FW105 a lot yet? I think the Skate 108 would make sense as it's considerably larger than the other 2..:-) but I would really suggest to try the FW´s first to see where the gap might be?
On the other hand, if you want to use the FW's as you say for mainly B&J and need a flat water board for basic freestyle, then the Skate 99 could be good if you don't mind the overlap in range to the others -the Skate 99 will plane earlier than the FW105 for sure, better for the basic freestyle and more advanced moves too? 6.2 is the max you should probably use on it? Of course not as early planing as your 108, but more agile for sure.
Do you have any other boards in your range that you might want to update as it does seem like quite an overlap having three boards so close together?