How can I add high and low voltage signals to a high voltage DC offset?
The answer depends on how acceptable it is for one signal to back-drive the others.  In the worst case, you isolate the AC signals each with their own series inductor and capacitor.  These would be narrow band filters to pass only the specific frequency (3 kHz and 20 Hz in your case).

The DC would be isolated with a "large" series inductor.  This would let the DC pass, but have sufficiently high impedance at 20 Hz to not attenuate that signal much.  The 3 kHz signal will see much higher impedance, so anything that works for the 20 Hz signal will work for the 3 kHz signal.