The VCS 3 (from Voltage Controlled for Studio with 3 oscillators) is a portableanalog synthesiser with a flexible semi-modular voice architecture, initially made in 1969 by Peter Zwinofeff 's EMS company. The electronics were largely designed by David Cockerell and the machine's distinctive visual appearance was the work of electronic composer Tristram Cary. The VCS 3 was more or less the first portable commercially available synthesizer. (Previous machines from American manufacturers such as Moog, ARP and Buchla had been large, cabinet-based affairs.) Significantly, it retailed for just under £1000 in the UK. It was fondly acknowledged to be somewhat hopeless as a melodic instrument (due to its unreliable tuning) however it is renowned as an extremely powerful generator of electronic effects and processor or external sounds.

The VCS 3 has three Oscillators, a Noise Generator, two Input Amplifiers, a Ring Modulator, an 18db Voltage Controlled Low Pass Filter (VCF), a Trapezoid Envelope Generator, Joy-Stick Controller, Voltage Controlled Spring Reverb unit and 2 Stereo Output Amplifiers. Unlike most modular synthesiser systems which use cables to link components together, the VCS3 uses distinctive a patch board matrix into which pins are inserted in order to connect its components together.

The VCS 3 was quite popular among the progressive rock bands of the day and was used on recordings by Yes, Brian Eno (when he was with Roxy Music), The Who, and Todd Rundgren, among many others. A well-known example of its use is on Pink Floyd's Dark Side of the Moon.

The VCS 3, in spite of the fact that it is a monophonic synthesiser, underwent something of a renaissance in the late 1990s and early 2000s, with bands who looked to include a more retro synthesizer sound in their music.

It was also repackaged as the Synthi A (a version housed in a plastic briefcase).