I'm not sure I understand your question....
do you mean "What foods does my dog need to gain weight?"
And what do you mean by "Congestive heart failure -What if I made a different decision?" If you could give more details in your question it would help us understand. You can say as much as you wish.
Regarding your dog having lost weight in the last 3 days...has he/she been eating as much as normal? And lost weight anyway? Or has she/he had no appetite at all? Has the vet told you what stage of CHF your dog has?
You could certainly try some home cooked meat with a small amount of vegetables, and add some cooked potato or rice. That might be more tempting than plain dry dog food right now.