Perhaps you could get round the small lathe problem by machining it in separate pieces? Machine the ends you require on something shorter, then join it to the steel bar (e.g. press fit, key, etc)? Unless the runout required is really strict (seems unlikely due to the length), that should be fine...

Or just find someone with a not tiny lathe. Mine has a 38mm spindle bore. Not sure about 3m long though - there may be a wall in the way!