This is my recommendation for carrying a canoe and two kayaks on your Spinter van and loading them easily...
1) Install the Thule 387 Roof Rack system to the Dodge Sprinter 3500 van's raingutters. This is a hi-rise setup that will clear your roof nicely. You won't need an extra set or anything. You can simply spread out the distance between the 2 loadbars so you have a solid platform for carrying long items like a canoe and kayaks. Choose the 78inch load bars length too, as this will ensure enough space to attach the gear carriers.
http://www.orsracksdirect.com/thule-387-complete-roof-racks.html
2) Attach the Thule 579XT canoe carrier in the middle on the load bars. You can then just lift the front end of the canoe onto the rear Thule 597XT cradles then slide the canoe forward into position.
http://www.orsracksdirect.com/thule-579.html
3) Attach a Thule 897XT Hullavator kayak carrier system on both sides of the load bars so you can easily load the 2 kayaks from the side.
http://www.orsracksdirect.com/thule-897xt-hullavator-kayak-rack.html