I've built start-ups both web/app-powered and people-powered. You've gotten good advice from the others so here are some of my top-of-mind thoughts:
1. Do integrate your 2-3 year plan with your 1-year plan.
2. Don't over plan for Years 2 and 3. Just lay out a few (not all) possible futures with an estimate of the order of magnitude of the potential size your business in some basic business terms:
a) revenue and where it's coming from
b) customers or members or audience...depends on your business model
c) when do you expect to make a profit (if ever - which is okay depending on your business model)
d) if you plan on selling the company, who the possible candidates would be (e.g., big company in your space, the public, etc.)
e) what and how many resources will you need and where will you get them? It's a "sellers market" for talent right now so this will be your Achilles heel.
f) can you lead the team
3. Do plan Year 1 in concrete, achievable milestones that support the theory of your business case...both your business model, what problem your web/app, and does it actually work.
4. Do build a start-up team that will get you through the first 90, 180 and 365 days with an eye toward years 2 and 3.
The level of detail and polish of your plan will depend on your audience. If it's for yourself (and your own team), don't over do the packaging but do make it very concrete. If the plan is for outside investors, do everything you'd do for yourself but do package it for external consumption...but make it short and sweet.
Please Request a Call if you'd like to discuss further.
All the best.