Here is the link that explains the process:
In either event, the best way to approach an appeal is to ask a Realtor to assist you with selecting the most accurate comps and then include that info in your appeal. I've assisted a number of people with this, so feel free to email me at email@example.com and I can get to work.
Best way is to have a Realtor do it for you. I do this for my clients all the time at no cost. If you like, you can contact me directly and I'd be happy to provide you with comps to appeal the taxes for your properties.
As Eric said look at the assessors site on sfgov.org. My experience is that a letter from a Realtor is worth more than the home owners opinion. You can start any where you want but you might want to contact you agent and have them produce a comparable market analysis. They can also write a letter of value much like we do for estate planning purposes and other base value setting situations.
There is an agent in my office that used to sit on the tax assessement appeals board, now he works with people when they are appealing to the board.
If you haven't done it yet, you should visit the assessors portion of SFGov.org They will be able to explain the process of appealing your property tax base. There are also deadlines, so you should check right away to see if you've missed the deadline for 2010-11 year taxes. My guess is that you have.
As far as value is concerned, sounds like you've already identified the 3 comparable sales you need. I'm not sure which the assessor will latch onto-likely the higher of them.
Hope that helps somewhat.