It was designed to provide features business users wanted: a true typewriter-style keyboard with upper and lowercase letters (the Apple II only supported uppercase at the time) and an 80-column display.

Unlike the Apple II, the Disk III controller is part of the logic board.

Special chips were intentionally added to prevent access from Apple II Mode to the III's advanced features such as its larger amount of memory.

In order to do so, an emulation boot disk is required that functionally turns the machine into a standard 48-kilobyte Apple II Plus, until it is powered off.

The keyboard, internal floppy drive (and one external Disk III), display (color is provided through the 'B/W video' port) and speaker all act as Apple II peripherals.

The paddle and serial ports can also function in Apple II mode, however with some limitations and compatibility issues.

Similarly, access to lowercase support, 80 columns text, or its more advanced graphics and sound are blocked by this hardware circuitry, making it impossible for even skilled software programmers to bypass Apple's lockout.

A third-party company, Titan Technologies, sold an expansion board called the III Plus II that allows Apple II mode to access more memory, a standard game port, and with a later released companion card, even emulate the Apple IIe.

After overheating issues were attributed to serious design flaws, a redesigned logic board was introduced in mid-December 1981[3] – which included a lower power supply requirement, wider circuit traces and better-designed chip sockets.

[15] The 14,000 units of the original Apple III sold were returned and replaced with the entirely new revised model.

[7] Former Apple executive Taylor Pohlman stated that:[18] There was way too short a time frame in manufacturing and development.

The case of the Apple III had long since been set in concrete, so they had a certain size logic board to fit the circuits on ...

Basically, customers were shipped the pilot run.Jobs insisted on the idea of having no fan or air vents, in order to make the computer run quietly.

One advantage to the aluminum case was a reduction in RFI (Radio Frequency Interference), a problem which had plagued the Apple II series throughout its history.

Unlike the Apple II, the power supply was mounted – without its own shell – in a compartment separate from the logic board.

The decision to use an aluminum shell ultimately led to engineering issues which resulted in the Apple III's reliability problems.

inCider stated in 1986 that "Heat has always been a formidable enemy of the Apple ///",[13] and some users reported that their Apple IIIs became so hot that the chips started dislodging from the board, causing the screen to display garbled data or their disk to come out of the slot "melted".

The logic board used "fineline" technology that was not fully mature at the time, with narrow, closely spaced traces.

[22] When chips were "stuffed" into the board and wave-soldered, solder bridges would form between traces that were not supposed to be connected.

This caused numerous short circuits, which required hours of costly diagnosis and hand rework to fix.

Donn Denman ported Applesoft BASIC to SOS and reworked it to take advantage of the extended memory of the Apple III.

Apple III Microsoft BASIC provides double-precision floating-point variables, taking 8 bytes of storage,[23] while Apple Business BASIC offers an extra-long integer type, also taking 8 bytes for storage.

It was unfortunate the way things worked out, because we probably put $100 million in advertising, promotion, and research and development into a product that was 3 percent of our revenues.

Ed Smith, who after designing the APF Imagination Machine worked as a distributor's representative, described the III as "a complete disaster".

[28] By early 1984, sales were primarily to existing III owners, Apple itself—its 4,500 employees were equipped with some 3,000-4,500 units—and some small businesses.

[29] Wozniak estimated that Apple had spent $100 million on the III instead of improving the II and better competing against IBM.

[citation needed] At the start of the Walt Disney Pictures film Tron, lead character Kevin Flynn (played by Jeff Bridges) is seen hacking into the ENCOM mainframe using an Apple III.