Explanation

The judgmental mind behind this sentence finds his grandmother’s décor “vulgar,” so the first blank should be a synonym for that. Tacky fits the bill. For the second blank, you need a word that describes what it is not to tell your grandmother you find all her knick-knacks ugly.

Rancorous is the opposite of what you want, and doting, meaning “be extremely fond of,” goes too far. Tactful fits perfectly. For the third blank, you need a word that describes being nice to someone in order to get a good birthday present. Amicable doesn’t capture the strategy of it, but duplicitous is way too negative (it means “deceitful,” which is not really an apt description for the act of refraining from telling your grandma that she has terrible taste). Tactically, meaning “by way of showing adroit planning,” is correct.

Tacky, tactful, tactically.