Does MacBook Pro mid-2012 have a USB 3.0?

Yes. They do come with USB 3 ports.

MacBook Pro 13 inch Mid 2012

http://www.everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i5-2.5- 13-mid-2012-unibody-usb3-specs.html

Connectivity includes AirPort Extreme (802.11a/b/g/n), Bluetooth 4.0, Gigabit Ethernet, a Firewire "800" port, two USB 3.0 ports, a "Thunderbolt" port, audio in/out, and an SDXC card slot.

MacBook Pro 15 inch Mid 2012

http://www.everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i7-2.3- 15-mid-2012-unibody-usb3-specs.html

Connectivity includes AirPort Extreme (802.11a/b/g/n), Bluetooth 4.0, Gigabit Ethernet, a Firewire "800" port, two USB 3.0 ports, a "Thunderbolt" port, separate audio in and out ports, and an SDXC card slot.

MacBook Pro 15 inch Mid 2012

http://www.everymac.com/systems/apple/macbook_pro/specs/macbook-pro-core-i7-2.7- 15-mid-2012-unibody-usb3-specs.html

Connectivity includes AirPort Extreme (802.11a/b/g/n), Bluetooth 4.0, Gigabit Ethernet, a Firewire "800" port, two USB 3.0 ports, a "Thunderbolt" port, separate audio in and out ports, and an SDXC card slot.

I have a mid-2012 13" MacBook Pro also, but the hardware description in "About This Mac" says it's a USB 2.0 bus. Is there a difference from the 15"? Did Apple change over to 3.0 sometime around mid-2012?

Yes, it does.

(142023)

They all say USB 2 initially.

what does it say after you connect a USB-3-capable device?