All the three are free for small team.

Skiff

• Pros: E2EE, full-text search
• Cons: No edit-lock feature, team member may accidentally alter a doc.

Tettra

• Pros: Edit-lock feature, Q&A feature, multiple modes for ordering in tree (manual, A-Z, creation time, modification time, etc)
• Cons: Not E2EE, no drag-and-drop in category tree

Notion

• Pros: Popular, edit-lock feature, drag-and-drop to organise pages
• Cons: Not E2EE

My personal consideration

• No E2EE is acceptable momentarily
• Need edit-lock, drag-and-drop are quite important features; thus Notion should do.