You’d probably be able to do it well inside of a month (avg a few hrs a day), depending on how quickly you learn. I’d put this on either situation for #1. The first week or so should be spent making little things and working to expand your knowledge in the areas you need to know the most. Lots of tuts, lots of documentation. If your other 2 are going to be programmers as well, then you’ll have to learn all this for each, in addition to how to work as a team. If the other 2 are artists, they will need to learn their respective areas (obviously). In any configuration, you should be up and running with a prototype inside of a month. Not necessarily a presentation prototype (investor meeting) and very likely extremely rough around the edges. As far as multiplayer goes: know that there is a very particular way of doing things which will allow them to work in multiplayer and you won’t be able to just get any structure working.
That’s all assuming you want something you can play in editor and it doesn’t have to be anywhere near perfect. Some of the other replies are geared towards a serious undertaking, but I think you’re just wanting to dive in and get something working (even if it’s not best practices and will never see release). If you’re wanting to go after a publisher or other funding options, 3 months is a good target- just note that it’s better to delay your initial contact until it’s polished. Either way, I’d say a team of beginners won’t see much benefit project-time wise from working together. At least not within the first month or so.