Well, that puts us at 4.

Basic PMD lore is basically:

-humans don't exist, or if they do, they're basically fairy tales. You play as a human in a couple of the games, but you're turned into a pokemon for one reason or another.

-Pokemon can all conveniently speak English

-Pokemon live in towns and cities, just like us. They have their own currency, laws, trade, etc.

-there are still "wild" pokemon, but they're basically just either living out in the wild instead of cities or whatnot.

TL;DR You don't actually need to know the plot of any of the games, just imagine the world of pokemon but humans are replaced with pokemon.