Friedrich (Fridericus) III started his reign as king of the old German State of Brandenburg in 1688 and then, in 1701 became King Friedrich I of Prussia and ruled until 1713. Eric's coin is from the transitional years, so it would typically be described as a thaler (or fractional thaler, or numbered grosher) from Brandenburg-Prussia.
The fact that these coin have survived all these years is quite remarkable, and if you have one in good condition it is even more amazing.
Assuming Eric's coin is silver (I do not know of a gold issue like this) and in a condition approximately equal to our picture, figure a value around the $100 US dollar mark. There are many variations, and many coins with very similar inscriptions, so it is difficult to pin-point even an approximate value. A rare variety in superb condition would command several 100s of dollars. A small, copper coin like this with a lot of wear would be worth $20 or so.
Two web sites that deal with such material are:
and contacting these people will likely give you a good handle on precise identification and value. If you like, Eric, start an e-mail exchange with CoinQuest and send us pictures. We may be able to help you more.