I use 1 indoor, 1 grain free and 1 just whatever I found best.
Not gonna lie but the ones the UK facebook recommend Are not the best, the brands are okay, but the actual food they recommend from these brands arent that great !!
But then again I am pretty picky about what I feed Holly to.
For overall protein and fat the basic calculation is taking the % protein of each food, adding them together then divide by 2. However this is obviously for an even half and half mix.
I'm not good with maths so dont ask me how to work out if its not an even half and half my mum made me a spread sheet to do that
This is good to read to get the basics of the foods to pick good ones
However if you want to be even more picky then have a look at this one after reading the first
I personally would use the first two only out of all of those. And then look for a different one for the third food.
The main reason that mixes are suggested is so that if one changes if you loose one of the 3-4 suddenly its better and easier for the hog, rather than loosing just their one food.