You need to find the real needs of the real users and customers. You need these in priority order. Perfect answers are not required, just good enough answers. Now get your team focusing on getting things done in priority order. When your 3 months are up you can then say "we didn't get everything, but we think it is good enough".
If you cannot get someone else to tell you priority, then start asking questions until you can come up with an answer. Sometimes you have to make a decision and hope you are close enough. Sometimes you will find other people (marketing, sales, your boss) who will help. An incomplete team that recognizes that things might not be perfect but they can influence one where the imperfections are is better then going alone, but go alone if you must.
Note that customers and users are not always the same! If it is a customer need you can fake it as it won't be used, so make this look good, but make it obviously useless to users. If it is a user need it needs to work right, but it doesn't need to look as good since the customer has already bought it. Of course for release two you have feedback on what needs to be fixed.