Usually techs just estimate the amount of refrigerant they put in. The proper way is to weight their tank before and than weight it again after the install, nobody bothers to do that.
The tech probably made up the figure of 3 lbs. figuring you don't know enough to check your system's label.
Any tech can check if they have installed the right amount by checking the proper pressures on your unit. I have noticed they do often overfill.
Call the owner of the company you had come out and tell him what is going on and ask to have your system's pressure checked. They can perform a simple formula to get the proper level. This is called superheat and subcool.
http://www.contractingbusiness.com/2...se/17963/issue (I am trying to post a link to a site about superheat but I don't think this forum will let me. You will have to search this up yourself)
I always recommend checking the A/C company with Better Business Bureau. And if needed file a complaint. This usually gets their attention.