Why 3-tier?

Unfortunately the 2-tier model shows striking weaknesses, that make the development and maintenance of such applications much more expensive.
The complete development accumulates on the PC. The PC processes and presents information which leads to monolithic applications that are expensive to maintain.
In a 2-tier architecture, business-logic is implemented on the PC. Even the business-logic never makes direct use of the windowing-system, programmers have to be trained for the complex API under Windows.
Windows 3.X and Mac-systems have tough resource restrictions. For this reason applications programmers also have to be well trained in systems technology, so that they can optimize scarce resources.