Cross chaining is so overblown these days. Modern bushingless chains are more flexible than the older chains. Shimano “requires” cross chaining on all of their 1x11 setups. If you do the math it is nothing more than how far off chainline (chain angle) you can ride within manufacturers specifications. Shimano 11 road is 18.7mm off chainline and 11 mountain is 19.5mm. End of cross chaining rant.

As for your 3x7 setup, with a triple I ride the number of cogs minus 1. Once again it is simple math. If I can ride all 7 the cogs in the middle chainring, (shifter position 2), then when I move the chain over 5mm to a different chainring, I move the max cog over 5mm; which on a 7 speed is one cog. It is still about the chain angle.

Technically if your chain length is correct, and your rear derailleur cage is long enough, you can ride any combination for a short distance, but for everyday riding.

Large chainring: 1-6
Middle chainring: 1-7
Small chainring: 2-7

This is based on 1 is the smallest rear cog and 7 is the largest cog.

John