Those third act deaths are either the Great Glorious Come Uppance or the Ghastly Gutting of the Soul.
Yes, I usually know who is going to kick it in the third act from the beginning of the crap draft. That character owns the burden of pushing the protagonist the hardest. Their death symbolizes the Goal Achieved. Sometimes it's a merciful death or the final sacrifice. Other times it's hubris.
The trick is balancing the deaths.
Too many and the reader doesn't care; they're inured. Too few and the story rings hollow; after all, the value of life is driven by the inevitability of death. Not every death has to be gruesome. Not every set up for the dying should be intricate. Not everyone has to die by the antagonist's or the protagonist's hand. Oh, and don't overlook a good maiming; it can deliver equal--or better--emotional resonance.
All that said, if you're reading one my books, someone will die, folks. Usually a lot of someones.
However, I guarantee you, it'll never be the dog.