-- Last Updated: Jan-21-13 3:21 PM EST --
I have no expertise in the issue of building a kayak, especially a triple. The one thought that came up was that you might find yourself very challenged turning such a craft, with only one adult aboard, if the wind kicks up on you. It has a habit of doing that from early afternoon into early evening over ocean water. So planning and timing could make a huge difference in safety for you if you are thinking of getting to places like offshore islands.
My other thought was the rescue scenario. I happen to agree that kayak over canoe is going to be better for the open water you describe, on windage alone, but question being out there with just one boat. I am wondering how you and your sons would handle being in the water at one time and having to get back in. Are they solid enough swimmers etc that you could somehow get them back in either by getting in yourself first, or by throwing them into the boat and keeping it steady while you climbed back in?
It seems that you really have to go them first or them last. You first means you are trying to manage a quite long kayak to get to each of them in the water if they don't hold onto the thing really well. You last means that they are trying to manage it while you climb in. In conditions likely to cause a capsize to start with - it is always a surprise! - both of these could be difficult.
Having two boats, perhaps you with the 5 yr old and the slightly older one in a craft of their own, would leave you (literally) more resources to handle a surprise. You can always tow the other boat if it becomes an issue of tiredness. And it is usually easier to get someone back into their boat from being in your own.
If you were talking lakes and near shore, this stuff matters less. But if you are talking about going well offshore on open water, I would have some real concerns about having only one boat.