If you ruled them as separate attacks, then they'll get their Impervious, Invulnerable, and Toughness taken in to account for each hit. In Eric's scenario #2, you'd actually do:
0+0+1+2+3 = 6 on an Impervious or invulnerable person, if they didn't make the Impervious roll.

I wish the rule of 3 didn't apply here, though.

Well, on the bright side,V Flash can outwit V Rhino's Imvul and roll a 4,5,6,7...not too difficult...to hit for four damage, that's not too bad.