***SYNC SD3 TO AUDIO TRACKS***

But you do know the tempo of the audio, right, if you've created it in RealBand yourself? If so, simply set the project tempo to match that tempo already before you import the audio.
In order to replace the drums with SD3, you need to convert it to MIDI, in case it is not MIDI yet. I've never done that in SONAR, but I think/IIRC V-Vocal should be able to do it reasonably well, as drums is the easiest instrument to convert to MIDI.
 
If the drum track is MIDI already, you simply use that MIDI track with SD3. I don't know if the mapping (i.e. if the keys respond to same kitpiece) matches. If it doesn't, you need a drum map for RealBand, or simply move the notes on the right keys in Piano Roll (that only takes a few minutes).