How to route multi midi input channels in one instrument plugin

[RufusDrums]

Hi there!
I'm using the Arturia B-3 V2 plugin which supports multi midi channel inputs to use more outboard gear to trigger different parts of the plugin (e.g. Bass Foot Pedals on Midi Channel 1, lower keyboard on midi channel 2 and upper keyboard on midi channel 3). I managed to set up three different keyboards to send midi on three different channels for the corresponding parts in the plugin. But unfortunatly ableton mixes all the diffenrent input signals back to channel one so just one part of the plugin gets triggered.
Is there any workaround so that ableton sends alle the different midi channels incoming on one track correctly to the different midi channels in the instrument plugin?
I don't understand why ableton merges the different midi channels in the first place!?
Thanks for your help!
Fiete