In my opinion, in Stranded III will be singleplayer and multiplayer i'm not sure about multiplayer), and I think will be able to play Stranded III in internet only with Unity3D.
I don't know, but I want to know: Will be multiplayer in Stranded III?

Yes, the development blog and the thread used for comments says it will be multiplayer, you should've asked on the comments thread rather making a new thread about it.