This goes for both 3-Hand whist mainly, but also good to know for other games as well.
Before starting a game of Bridge, all players count up the quality of their cards. Since Bridge is based on Whist, this goes just as good here. Here is a way to find the quality:

1)
Give all your
Aces 4 points
Kings 3 points
Queens 2 points
Jacks 1 point

This way, a deck of cards will have 40 points. In 3-hand whist, where you can choose to change your cards, this is very handy. If your value is lower than the average of 10 (if you deal 4 hands with 13-cards, the average value of each hand is 10 point), you really should consider to swap.

2) Count the lenght of trump. In a deck, there is 13 cards. Devide it into 4 hands, and an average hand has about 3 cards. If your trump is 0,1,2 or 3 cards, you should also consider to swap. Of course, if you have ace,king and queen in the trump, it is more than enough

3) Count the number of tricks you think you can take. is the number 3 or lower, then swap.

Combining these 3, you should up your chances in this game.