I've read that the Mars InSight Seismometer will be deployed before the HP3 probe. Won't the digging interfere with the seismometer? Or the shock produced will be small enough to be dissipated by the ground?
As shown in the graph below, the digging of the HP3 probe is expected to occur between sol 50 and sol 90. This gives SEIS plenty of time before sol 50 and after sol 90 to take measurements. Furthermore, the graph shows that the actual time spent hammering is quite brief. Finally, although the hammering will indeed interfere with measurements of ambient seismic activity, the echoes from the hammering might reveal information about nearby subsurface geology, which is also valuable.
The digging may produce a seismometer reading, but that's not necessarily undesirable. They know exactly how the digging operation will be performed, so it's an opportunity to check the seismometer.
The digging is part of the experiment:
"As the heat probe makes its way down, its hammering motion will generate vibrations. Scientists back on Earth can use the seismometer to detect these vibrations and study the ground beneath. As the heat probe hammers down, the seismometer should be able to measure the speed of the vibrational waves in the soil generated by the hammering motion. It should be able to detect reflections of these waves as they bounce off any shallow buried layers, such as lava flows, unveiling the structure of the subsurface"