If Session Drummer appears to trigger the wrong instruments either the change the MIDI note sent by the Alesis or create a Drum Map in SONAR to translate the incoming MIDI note to the note Session Drummer expects. The MIDI note mapping for Session Drummer is documented in the Session Drummer help, "Using Drum Pads" section. Here is a link to the X3 help on creating drum maps
http://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=DrumEditor.03.html. You will need to refer to the Alesis manual to change the MIDI data on that end, I do not have the device.