The Third Hand should not bid a No-trump over the Dealer's Heart or Royal, unless he have the three remaining suits safely stopped, or his hand contain solid Diamonds or Clubs, and one other suit stopped.
WHEN "TWO HEARTS" OR "TWO ROYALS" HAS BEEN DECLARED
The declaration of two Hearts or two Royals is practically a command to the partner not to alter the call. It indicates at least six sure tricks, probably more, and a valuable honor count, in the Declarer's hand, provided the suit named be the Trump. The Third Hand should only change such a declaration when convinced beyond reasonable doubt that his holding is so unusual that he is warranted in assuming the responsibility of countermanding the order that has issued.
Weakness in the Trump and strength in some other suit is far from being a sufficient justification, as the chances are that the Dealer is weak in the suit of the Third Hand, and called "two" mainly for the purpose of keeping it from being named. To overbid two Royals or Hearts with three Diamonds or Clubs is obviously absurd, unless holding _five honors_ and such other strength that game is assured.
To overbid two Hearts with two Royals, or two Royals with three Hearts, is almost tantamount to saying, "Partner, I know you are trying to shut out this declaration, but I am strong enough to insist upon it." Such action is only justified by 64 or 72 honors, and a sure game.
To overbid two Hearts or two Royals with two No-trumps, as a rule, means 100 Aces. High-card strength assures the game in the partner's call with probably a big honor score; only the premium of 100 makes the change advisable.
With strength, in the case under consideration, the Third Hand should advance his partner's call with much greater confidence than if it were an ordinary bid of one. He should not worry even if absolutely void of Trumps; in that suit his partner has announced great length as well as commanding cards; Aces and Kings of the other suits are what the Declarer wishes to find in his hand, and with them he should bid fearlessly.
The same line of comment applies with even greater force to the action of the Third Hand when the Dealer has bid three Royals or three Hearts.
When the Dealer bids one No-trump and the Third Hand holds five or more of any suit, one of the most disputed questions of Auction presents itself.