Stellaris > CK3 > Vic3.
At least until the damn slackers update Elder Kings mod then CK3 will be best.
Vic have way too much micromanagement for me. Stellaris have most content and best mods, but it bloats like crazy and work like shit later in game.
Problem with pirating those games is that all of them live by the mods, and best source of mods is Steam workshop. I think CK3 is the one best here since major total conversion mods usually have github or other alternative download. So my advice would be pirate this one and see if you like it.
Finally there is DLC question if you want to buy. Vic is the newest, it have only single DLC but is pretty barebones overall. Stellatis have shitton of DLC so it's most polished with most content, but it would be expensive, though you can get a lot of them very cheap in keyshops or whatnot. CK3 is middle way, last DLC about tournaments was banger, easily best PDX DLC ever for me.