extra wide tire liners?
 

My son has an Electra Straight 8 with 20"x3.0" 'Fatty-0' tires. I've had to repair flats multiple times (once it was both tires at the same time!) and ended up replacing both tubes. Each time I check for thorns (the usual culprit is about 1/4 to 1/2 an inch long) and pull all I find. I've been looking for 3 inch wide liner tape, but so far haven't found any. Does anyone know of a brand I can look for?

I know I can use Slime, but I hate that stuff, having had to clean it out of many valve stems.

Any other ideas? Believe me, my wife and I have looked all over for where he's picking up these massive thorns, and have no clue.

Not as wide as you are looking for but they might do the trick:

Spinskins
Mr. Tuffy Brown

I am not sure if they make thornproof tires in that size. You could try searching for those.

I imagine you already found the tire liners though and I did not extend your knowledge any, sorry.

Might have to use 2 liners per tire if you want that kind of coverage. The Panaracer Flat Away Tire Liners are 40mm wide or about 1-½" and come with adhesive on one side. Not sure what the widths are for the other brands or how you are going to secure the liners in position in the tire without using duct tape. Plus you are going to have to cut the liner to size, so a bit of rounding off any edges is mandatory. Would recommend just trying the Mr Tuffy Brown (one per tire) and see how that works out. Can also cut off the valve stems and use the old tubes as a liner or in addition to the Mr Tuffy liners.

Workman sells kevlar belted tires and HD tubes, but the tires are 2.125 wide instead of 3":
http://www.smallcarplans.com/Issue%2024.pdf

Mr. Tuffy's are the most popular liners and I would suggest buying those over spinskins or slime liners because of the reviews and experiences people have shared. Liners can cause flats too...the edges cutting into the tube or the liner dries out and begins to break or fall apart. Liners can last for years or fall apart in a few months. Do a search about the various liner brands before buying.