If you are just a casual tourist who wants to see some of the major sites and get the flavor of the city, I don't think a personal guide is really of any benefit--just get a good guidebook or two, check out the internet, and do your own research for general background of sights, and for city layout and how to get around. Three days in BKK is not a lot of time, so don't overestimate what you can cover, and due to traffic, plan activities geographically.

I would only recommend a limited-use personal guide for people who have a professional or special focused interest in a particular topic, and need more in-depth information than a regular guidebook can give...or need special access to places not normally open to the public. In this case, some advance planning and arrangement is necessary, and probably not with a standard travel agent. This does not apply to 99% of visitors to Thailand.