Ouch. That's terrible 3 people is not that large an advantage over one. There are triple the baddies already and you are all in the same base. The game stage should be averaged and maybe bumped a tiny bit... That would mean my Party game stage on Day 63 is 682 and solo at day 100 is 297. No wonder the game got so much harder by adding people. I can't believe being in a party makes it harder than not being in a party.
It makes the game easier.
In your singleplayer the only one player has to deal with gamestage 200.
With 3 players each gamestage 200 200you do not have to deal with a gamestage of 600, but only 480.
So you have tripled your e.g. damageoutput but get only 2,4 times the "quality" of enemies.
The difficulty in 7d2d is not based on the current day, but on the character progression. 3 player together progress faster of course. So comparisons like "singleplayer on day 100 is easier than 3 players on day 60" are useless.
That means if we don't party up for horde night the game stage would calculate separately for each us so we'd have 8 zombies coming at me at gamestage 176, 8 at 140 and 6 more at 140? Why does partying pump the game stage so much? Solo day 100 I can take 24 zombies MUCH easier than 3 of us can take 24 zombies at day 63.
Yep, that means if you do not party up every player will get his own gamestage but in total that's still more then partied up.
Only reasonable difference is, if the overal gamestage exceeds specific limits, you won't get demolishers in separate waves, because of low "gamestage per player", but in the combined gamestage you will, because it's a higher value.
3 players gives you not even twice the "stuff" as 1 player. Having twice the stuff doesn't let you make a base that kills twice the zombies, yet you have triple the zombies, and they are much stronger than if you were alone. I don't understand that at all.
No, you don't find twice the "stuff", but you are still more players you can do triple the amount of whatever. However you indeed get more loot with 3 players. You can either decide to loot one POI as a group, which makes it faster and safer. You can carry triple the amount and stuff them in a crate, optimize stacking and on the way back to the base be more efficient. Or you can loot different buildings in parallel, which is also faster. Usually we do big POIs like the factories or hospitals together, but when it comes to small POIs we spread out and loot different buildings. No matter which way, it is much more loot in the same time.
You seem to expect that the difficulty should not change when adding more players, but then it would become way to easy.
You can adjust the game to become easier in multiplayer. Lower the XP-ratio to get slower progression. Increase the loot drop rate to find more "stuff". Nobody forces you to use the same game settings for single- and multiplayer.
Lets say demolishers will appear above gamestage 300. In singleplayer you have gamestage 300, demolishers appear, but you have to deal with them ALONE.
With 3 players, everybody has to have "only" a gamestage of 125 (not 100!) before demolishers appear. Each payer is lower skilled then, has worse equipment, BUT then you are three people that can deal with the demolisher TOGETHER.
Or in other worde: Singleplayer is not easier, it just takes longer to reach the same gamestage.
BTW: it's gamestage, not gamestate.