I was thinking that we could do that through GPIB:
- acquire the spectrum (READ:ARRay:SPECtrum:result:CURRent?)
- process a peak search on acquired data
- set markers on the 3 main peaks
But unfortunately, it seems that there is no remote control command to set a marker (except FETCh:SPECtrum:MARKer:PEAK? for the R -> Peak).
According to the Operating Manual, the commands FETCh:SPECtrum:MARKer:ABSolute? and FETCh:SPECtrum:MARKer:RELative? do not affect markers in manual control.

Michel.