In Global, select the MIDI tab, then go down to "MIDI Clock Mode" and change it to "In No S/S" then press the GLOBAL button again to save your selection.
Now your MIDI sequencer will be the clock source, but the Pro 3 will ignore start/stop messages.
Note that if the sequence mode is set to "Trigger" and there are incoming MIDI notes from the external sequencer on the Pro 3's MIDI In channel, then the Pro 3's sequencer will be stepped regardless of the clock setting. This is because the "Trigger" mode works independent of MIDI clock. If this behavior is not desired, either (1) set the sequencer mode to "Normal" or (2) assign the Pro 3 its own MIDI channel, depending on whether you want to take advantage of the "Trigger" mode.