N/V

I do killer sudoku puzzles frequently. Having the combination sums handy let's me focus on the puzzle. For the last couple of years, I have planned on building an app for that. When I sat down to start last week, my research led me to 12 in 3, which had EVERY element of my design. No reason to reinvent the wheel. Well done. Well done.

The only thing I would change is to have the ability to cross off the combinations that don't work

Don't know whether I'll use it but it looks fine; What do you mean by "Allow double" (no game allows That) ?