Its long way to go, currently checking editor, its kinda different like Cossacks Vanilla Editor. When I make my own map could it be played in Multiplayer?
You cant really make online multiplayer maps unless the mods make it possible. So yes you can do it. No you cant do it with vanilla.
I wonder if the ingame editor that can be accessed from the main menu has alot tools hidden some way? As of now it only has the unit spawn tool, I wonder what the whole point of that editor is if you can only spawn units and not even select the player number?
You can change player with the num pad or Alt + 1, 2, ... 9, Peace time is Control + W, you can enable fog of war with Control + F and open automatically unit creation panel with P. But that's as far you can get with the ingame editor. For everything with terrain modification you need to use the editor at the launch of the game
You need to enter editor from Steam Library using RM on Cossacks 3 and Launch Game Editor. There are wide tools but I miss objects, like trees rocks etc. or I did not find em yet.