There are a few rare bridge players who avoid opening NT unless they have honours in all four suits. Most of us open NT with all balanced hands within the agreed point range. If we have a suit with three small there are a number of possible consequences:
The three small suit is not worth worrying about unless it is bid by the opponents. Now it becomes a danger suit. Because the odds increase substantially that it will be led, that your opponents will have most or all of the outstanding honours, plus they will more often have enough tricks in that suit to set your contract.
When the opponents bid a suit in which you hold Kxx or AQx, they have alerted you to danger and you would prefer to be the declarer. But in the absence of bidding by your opponents, a holding of Kxx or AQx does not signal a danger suit. In fact, there is an excellent chance that partner has a smaller honour opposite your holding and the contract is better played from her side.