The object after the minus sign in a place
The answer is a big hint for Which family member doesn't belong?
Please upvote pacoverflow's answer as well, without his help I would have never gotten to this answer.
The first part...
Guava fruit in a Bag - Baguava
The second part...
the U.S. state of Virginia (VA)
As a result...
BAGUAVA - VA = BAGUA
Bagua which are eight symbols used in Taoist cosmology to represent the fundamental principles of reality. This point is further enforced by the name of the puzzle (The mass of three) which eludes to the trigram (mass = grams/ 3 grams = trigram).
this is also a large hint for Sensoray's other puzzle
After 3 days, I think it's time for a partial answer.
The fruit in a bag is a:
The thing on the right is:
the U.S. state of Virginia (VA), horizontally squeezed
I'm not sure the bag and the horizontal squeezing are relevant - it might just be that the OP wanted to prevent reverse image searches.
GUAVA - VA = GUA, which is 3 letters and could fit the title.
What is "GUA" though? Perhaps the letters need to be rearranged to "AUG", which is short for August, and that could be the hint to the puzzle linked by OP, which they indicated may be related to astrological signs.
Could it be
Groceries - gross(goo/slime) = erie(s)
It might hint at the great lake Erie?
Going out on a limb here, but I really like this puzzle.
Lime in a bag = LMNABAGMNUSFR
'-' = MINUS
Fire = FIRE
And so the reasoning..
The letters that belong among all 3 : IE : Therefore the rest are, to your hint, not belonging
Further reasoning for answer :
I was more focused on the
Rebustag I think