When you attach the leads for 3 lead on the LP 12, it is not diagnostic quality, it is "monitoring" quality. The only time you get diagnostic quality is if you run a 12 lead.
However, if you attach the leads for the 3 lead, press the 12 lead button (it will warn you that you do not have all the leads attached), and then press the 12 lead button again to override the warning, it will print a diagnostic quality 3 lead. It will be on the full 12 lead grid, minus the missing leads.
This can help filter out some of the artifact, as the machine has a different algorithm it uses when doing diagnostic quality versus "monitoring" quality.
One of the reps showed me this a few years back, I thought it was an interesting trick...