These cords would be very low amp cords to hook up an element that may be 3 to 5 strings. That is violating the "3 string" rule however. I had 42" trees with 5 strings on each with this type of setup.
Each string is basically 2, 50 stringers together. You have a 22 gauge wire running down the thing on each end with NO lights attached to it. Simply pull the strings apart and find those wires (one on top and one on bottom of the stringer). You will have to cut and make a couple of splices. I ended up putting a dab of solder at that splice as well as taping it up. After making a few splices, I had a 22' extension cord with a 3 amp fuse in it!
I ended up using 16 "dead stringers" with lights pulled (so half wouldn't light up) last year on some of the mini trees and arches. I know this works well, because it worked with my setup through TONS of rain. Yes. We had some GFI issues. If they were from the before mentioned...taking the lights out will help. Also keeping the strings off the ground (project for next year!) will help as well.
I may end up looking at this for next year. I am contemplating the use of MR16's or some other type of flood lights. The reason for this is to possibly do away with the C9's incandescents that I am using now. They were a HUGE amp killer that I really need to change out to LED retro fits. However that will be an outlay of at least $100.00 on sale. With more limited funds this year on the display...that might be an issue. The floods (would be) and the C9's were used to highlight the house which is in the back of the display. The MR16's or other type of flood lights would require more channels, but might be more economical to run in the long run. I may have to buy a DC controller to use with this idea. If I do, I could actually do more "stuff" with it (with the floods) than with the C9's. Would love to do both....we'll see.