Are the 40 pc the same or they are all different ? Are all the people working on those 40 PC doing the same type of job or they are all different ? Why 3 servers ? Do they do different things ? Any network connection to the outside world or other parts of the company ? How many different kind of services running on these servers ? Are there custom applications on these PC's and servers ? Are there web services on these PC's and servers, and who looks after the web pages ?
The more complicated the office is job wise, the more people you need.
You cannot base number of IT staff just using number of PC's and servers. The complexity of the work place is way more important. And, of course, how do yo want down time handled - wait for couple of weeks for you to find someone to fix things, or they need to be up right now ?
Oh, how good are these people using those 40 pc's. Are they quite self-sufficient or do they need a lot of hand holding. Or are they the kind that will bring their own favourite software to the office and install them on their own machine, and spread them around to the rest of the office ?