data must be used exactly as published, down to the components and exact brand or type of case, primer, powder, wad and type of shot.
Incorrect - there are clones of OEM wads that are perfectly acceptable to use. A load for any of the Remington hulls can be used in ANY Remington hull. There are primers that are equal (or so close as to be equal) that are also acceptable to swap. Shot (talking about lead up to size 6) can be anything. For target loads, I use reclaimed which is a mixture of 7.5, 8, 8.5, and 9s. You are correct about not working loads up and slugs and buck using different data.
besides the two websites from the powder makers, the ABCs and Lyman have good info regarding duck/turkey/HD loadings
OP, you will also need different equipment since shotgun shell reloading is not done on metallic presses with any degree of success. Since you mention both 2-3/4 and 3, you will want to look at a MEC at the very least as it can be adjusted for different hull length.
The best 12 gauge hulls for reloading are Remington, Winchester AA followed by Federal Gold Medal in 2-3/4. The other Winchester and Federal hulls, especially the cheap wally world promos are NOT worth the trouble.
Generally, Remington and Winchester AA hulls share the same wad design, whereas the Federal does NOT.