If I did not finish some of the quests in chapter 2, can I go back in chapter 3 to finish them?
The "Lost Lamb" quest starts in Chapter 2 and is not intended to finish until Chapter 3, so you have to wait until then to finish it.
The "Dogcatcher of Vizima" quest can be completed in either Chapter 2 or Chapter 3, whenever you like.
You should definitely go to the Mage's Tower in the Swamp right away and talk to Kalkstein, to pick up your 1000 orens reward for opening the tower and to finish off that quest. The mirror in Triss' bedroom will teleport you directly to the Mage's Tower if you click on it.
The poker dice quests are mostly not time sensitive, and you can advance them at will, except for the legendary poker player you meet at the very end of the quest.
Most of the other quests are chapter specific.
And, although the player doesn't always notice this, the area files for Chapter 3 are completely separate from the area files for Chapter 2. So it's a different Swamp and a different Temple Quarter, even though they look much the same. The bad news about this is that if there's anything you dropped in Chapter 2, it's GONE. The good news about this is that if there's any loot you liked in Chapter 2, you can pick it up again in Chapter 3.