I've got a Furnace Scamp attacking my opponent, and it's getting through unblocked. I cast Tainted Strike on the Furnace Scamp so that it's now a 2/1 with Infect.
When the Furnace Scamp deals combat damage to my opponent, my opponent gets two poison counters. However, I choose to sacrifice the Scamp for another 3 damage... but will that mean that my opponent will get three more poison counters from that damage?
Correct. The Scamp is the source of the damage, and LKI tells the game that the source had infect. End result is an opponent with 5 Poison Counters more than they had before.
When the Furnace Scamp deals combat damage to my opponent, my opponent gets two poison counters. However, I choose to sacrifice the Scamp for another 3 damage... but will that mean that my opponent will get three more poison counters from that damage?
The latest Comprehensive Rules are also good, and can be found here.