Edit: I just realized I misread the question and answered a way more complicated case than what you asked. In the case where you literally already have three tricks and the other team has one, definitely throw it in. My answer below is for the more complicated case where you know what the final score will be without having played enough tricks to actually decide it
It depends on the group, and it can get complicated. You can be in a situation where you each have one point and you know that you can win two of the next three (without knowing which two), and in my normal group that plays all the time showing your hand is generally fine, since everyone else can figure out pretty fast how you were guaranteed to win. In a new group I've only played with maybe three times I did that and there was mass confusion and I ended up spending about a minute explaining why I was guaranteed to win, which is much longer than it would've taken to just play the hand (and probably just annoyed people)
As long as it's a friendly game and it's obvious to everyone why it's a win I would do it; if you think you'll need to explain it I would probably just play the hand out anyway, unless you're trying to teach someone how to tell that a hand is won