With a bid of Three, you're going to get to discard 3 cards from your hand. You will get to keep the cards you discard so you may want to discard point cards or you may want to try to "short suit" your hand.

For example, if you have 5 Tarocks, 4 clubs, 2 diamonds and 1 heart, you may want to discard the diamond and the heart. This will allow you to trump diamonds & hearts every time they are lead, giving you a chance to win the Kings and other point cards of those suits & give you the lead.

Knowing when to keep the lead and when to relinquish the lead is vital and is learned through experience. Generally, keeping the lead is one way of controlling the game and it means you're winning tricks, which is good!

Back to discarding; it would be ideal if your two diamonds were the Cavalier and Queen and your heart was the Queen. You're not only going into the game two suited (with only clubs & spades), but you're burying approximately 11 points for your side. You're well on your way to getting the 35 points needed to win your bid.