3x is the second revision of the Evolution-Data Optimized (EV-DO) set of mobile telecommunications standards made by the Third Generation Partnership Project 2 (3GPP2). It is also known as TIA-856 Rev B or EV-DO Rev B, antecedent to EV-DO Rev 0 and EV-DO Rev A.

It is one of several types of the CDMA2000 set of standards, its full designation being CDMA2000 3xRTT, where "3xRTT" stands for "three times Radio Transmission Technology" (three times because it has three carriers, in comparison to 1xRTT which has only one carrier).

The 3x protocol, like all other EV-DO standards, is a form of 3G technology. In order to maximize its rate of data transmission, 3x makes use of varied multiplexing techniques like frequency division duplex (FDD) and code division multiple access (CDMA). To achieve higher rates of data transmission, 3x may make use of two 3.75 MHz channels, or multiple 1.25 MHz channels.

Since 3x is a multi-carrier version of Rev A, it is also sometimes referred to as "Multi-Carrier" or "MC." Its multi-carrier specification gives it a number of improvements over its predecessor. It has an increased maximum downlink rate of 14.7 Mbps (4.9 Mbps per carrier for 3 carriers is expected for typical transmission).

Expected peak rates for a 3x network with two channels are 6.2 Mbps for forward link throughput and 3.6 Mbps for reverse link throughput.

For one with three channels, expected peak rates are 9.3 Mbps for forward link throughput and 5.4 Mbps for reverse link throughput.

Interference from adjacent sectors was reduced and rate was improved by hybrid frequency re-use.

Latency was reduced by using statistical multiplexing across channels, enabling new services like video telephony, web browsing, gaming and remote console sessions.

Services such as high-definition video streaming were also enabled by bundling multiple channels together.

Talk time and standby time are also increased. 3x also provided ample support for services that required different data rates for uploading and downloading, such as delivery of broadband multimedia content, transferring of files, and web browsing.

Upgrading a Rev A system to 3x is fairly inexpensive, and in some cases, upgrading from Rev A to 3x does not require additional hardware, as all upgrades may be applied in software.

At present, 3x has not been deployed and is not under development.