in os 3.0 we can connect midi contoller to montage through usb port .

New in version 3.00 you can connect a MIDI Controller that only has USB for MIDI to the MONTAGE “To Device” port, specifically.

Can we connect two midi controller using usb hub??

... if you mean to the MONTAGE, no.
But here is how you can (and always could) connect multiple controllers to the MONTAGE.

Method 1 — using a Computer DAW to merge
Connect your MONTAGE to your computer using the USB “To Host” port and install the Yamaha Steinberg USB Driver. In a DAW, like Cubase, go to STUDIO > STUDIO SETUP... > MIDI > MIDI PORT SETUP... In the column labeled “In ‘All MIDI Inputs’” place a mark next to each device (Controller) you wish to use as an input device to a Cubase MIDI Track.
_To make the MONTAGE a MIDI Input device, place an ‘x’ next to MONTAGE Port 1, if you want to merge a second controller thru MONTAGE also place a mark next to MONTAGE Port 3. (Do not mark Port 2 —it is used expressly for Remote Control). MONTAGE Port 3 allows you to connect a MIDI controller thru the 5-pin MIDI jack on the MONTAGE back panel. Data will be echoed back thru the MONTAGE acting as Midi Interface.
_Connect any other device you wish to merge to the computer via its own connection. If it is setup as a MIDI Input you can route it wherever you need.

Next, create a MIDI Track and set the MIDI IN to “All MIDI Inputs”. This will allow all of the devices you marked as a MIDI Input to act as controller to the target (MIDI OUT) device, when the Track is active. If you set the target (MIDI Out) of the Track to MONTAGE Port 1, it will reach the MONTAGE Tone Generator. If you set the target to MONTAGE Port 3, it will reach the device connected to the 5-pin MIDI Out.
Set the MIDI Out channel to merge all Track data to the selected Channel. You can have as many external controllers as you can connect to your computer... you just need to make them available via the “In ‘All MIDI Inputs’” column.

Method 2 — without using a Computer to merge
Purchase a 2-to-1 MIDI Merger, and use three 5-pin MIDI cables.

There maybe other ways to merge multi controllers to a target device... but this should get you started.