Answer honestly, if its a startup with only 3 people tell them that. If you have 30 employees tell them that too. Don't use any tricks or white lies. If you hire the candidate they will find out very soon how large your company is anyway. Yes, its disappointing if someone turns down your offer but it will be much worse if you hire someone and they immediately quit, or are disgruntled you lied and work poorly or make trouble.
This is one of those cases its best just to plainly and honestly state the truth.
Edit for updated question: Judging future opportunity is something only the candidate can decide. You cannot truly prove if your small business is a good opportunity or not. However adding any information that would suggest your company is growing, or other openings you plan to fill soon might help. But do not lie about this. If you are a 3 man shop for the foreseeable future say so. Some candidates desperately need a stable job to provide for dependents, and will not be convinced to work for a small business ever. Some might just want to know if there is a promotion track for them. You can't guess why they are asking so give them as much truthful information as you can so you can both make a good and fair decision as to if your business and the candidate are a good fit.