Let's say I own a store and want a secure network for my employees, an open network for people in the coffee shop and another network that links customers to my website when they log onto the network. Can I do this from one router?
Some home routers support a concept of "guest network", where one router supports two wireless networks on one radio, where one network is bridges to the LAN, and the other is only NATed to the WAN port (the guest network). This may do the trick for you.
However, since both networks use the same radio, they will be on the same channel. I think it will be easier to maintain and more straight forward to just buy two wireless routers, they are so cheap compared to a labor cost needed to maintain some non-trivial setup. Also, this allows to separate them physically, e.g. "clients" router is in the shop, "employees" router in the back office.