You need to estimate the rent you will receive, then estimate your actual profit per year. For example, $4,000 per year (wait, that's before property taxes...). I do believe in many markets a rational individual will pay about the same for a share as a studio apartment...

Then, you need to estimate the difference in capital cost between the 2 and 3 bedroom, and the difference in property taxes (the difference in property taxes needs to come out of the profit (above)). For example, the extra bedroom costs $20,000 and the property taxes on a 3 bedroom is $400 more than a 2 bedroom, so ratchet the profit down to $3600 in this example.

Divide the actual profit each year into the difference in capital cost. This will give you an estimate of your rate of return on being a landlord. In this case, your rate of return (excluding appreciation/depreciation labor and aggravation, damages, if any) is $3600/$20,000 = 18%.

Whenever I rented rooms, I ALWAYS drafted a written agreement.