How about
SampleCloud1: (!PenDown nextIndexMod: 3) eq: 0
SampleCloud2: (!PenDown nextIndexMod: 3) eq: 1
SampleCloud3: (!PenDown nextIndexMod: 3) eq: 2
The good thing about nextIndexMod: is that it starts at -1 (unlike countTriggersMod:), so the first SampleCloud is only triggered at the first !PenDown.
BTW You can also use a Replicator and:
(!PenDown nextIndexMod: ?NumberVoices) eq: (?VoiceNumber - 1)
Best,
Gustav