Ok, just did it in training mode. You can pull off three hits but the last one will hit them while they are kinda being knocked back and they can attack back easily.
Though it doesnt always work, it depends on their damage.
All this information is on Mario, since he seems to be average of all the characters.
0-19 damage: You can pull it off, but expect to get hit because the last one hits them in the air while they land.
20-33 damage: Cannot do it, the stun time and the knockback make it so they aren't stunned long enough to hit with the third but the knockback is high enough to miss the third hit.
34+ Damage: The stun is now long enough to hit them with a third before, instead of doing a Dsmash, Dsmash, Utilt, perhaps a Dsmash x3, Up Air would be smarter, giving them a total of 43 damage instead of 33.