We started sailing 5 years ago when our girls where 4 and 5 on a smaller boat, and now sail on a 32 foot Islander with the girls at 9 and 10.
When moving to a little larger boat, our main concern was having "a space for everyone." (Dedicated berths) It was tough to find without moving up to a 40+ foot boat, which financially was not reasonable, but we finally found a boat we liked with dual quarter berths for the girls and a V-berth for us.
Having three, this would be more difficult, and as you have a boat already, not to helpful, but a space for everyone's things might still be important. A cubby, drawer, etc for each kid to put their personal things (books, toys, collected beach treasures, etc) which they can fill up with whatever they want is good. Our basic rules is as long as it does not smell, and they can store it in thir space, it can come aboard. I think this really helps mentally to have their own space, even if it is not a whole berth.
Another thing we did last summer was to ask the girls to think about changes they wanted while we where out on a trip. What do THEY want me to work on?
This took a while for them to really understand as they tend to accept things the way they are, but with a few suggestions, they got it figured out.
The biggest thing they wanted was padding on the "bonkheads", the part of the bulkhead the quarter berth runs through which they continually bonk there heads on (this term has expanded to be lots of head bonking items on the boat.)
Second was good reading lights in there berths. Some other things they can up with were lockers that come open when and dump stuff while sailing, sticky drawers and doors, etc. Small things that I knew bugged me, but I could live with. I did not realize that some of these small items where issues for the kids too, and can make a big difference in there happiness on a trip whether they would have thought to mention them or not.
Stereo with CD player for music and audio books was a great addition for them as well.
Hope this makes sense and is at least a little helpful for you.
1964 Islander 32
Saint Helens, OR