Device has two DisplayPorts and each is powering an external display properly. It also has an HDMI port. Can I connect a third monitor here?
MacBook Pro with Retina display, iOS 6.1.4, 13" Late 2012
Applespec says up to two:
Everymac says theoretically three, but it gets WAY too hot:
2nd Display Support: Dual/Mirroring* 2nd Max. Resolution: 2560x1600 (x2*)
Details: *This model supports a simultaneous maximum resolution up to 2560x1600 on two external displays via Thunderbolt. Alternately, it can support a single display up to 2560x1600 via Thunderbolt and a single display up to 1920x1200 via HDMI. Although it can theoretically power all three external displays, as confirmed by a helpful reader, it runs too hot with three displays connected.
Also see: How many external displays can the Retina Display MacBook Pro models support? What is the maximum supported resolution of each? Are adapters required?