Expedition: general discussions.

The role-playing aspect is zero, or almost zero. There are zero events.

Well, then players will simply lose interest. After all, this is the most important thing in an expedition, isn't it? You can create bots and make them fight; the meaning won't change.
If something like this is to be done, it is necessary to partially replace the role-playing and game events with something, although I am not sure if this is possible.
For example, you could develop a complex turn-based combat mechanic where you need to find the enemy's weak spot and crush it, otherwise you won't win. But honestly, it's hard to imagine, and in that case, you would really have to implement a million different skills, abilities, and events, which would make the GM's and participants' work even more difficult.

The format is 3x3, meaning that tactical roles can be very diverse, as can the tactical possibilities for interaction between allies and opponents on the battlefield. Additionally, you can think of many more features, such as bosses (overpowered NPC opponents, but only one), obstacles, and generally non-standard arenas, etc.

Yes, I'm reading it in parts; that's correct.

Interest is maintained specifically by the combat, in a sense by the team spirit, and by achievements in the arena.

I'm not sure it will hold their interest. You could try to implement PvP or competitions between teams of players to see who is more successful in the arena, but I don't know...