I can pull it off a few times, but not as often as DB or Sly Bass. The input is 4653P, the 5 meaning you need to let the stick go back to neutral before doing 3P.
Another method I've seen people do is press H after the 46P then do 3P. The H basically freezes the input and the 3P will make the double palm come out. I remember doing this for that one combo training.