Let's do it!

Let the weight of mango A be a grams.

Let the weight of mango B be b grams and

let the weight of mango C be c grams.

Now the
Weight of all mangoes is
A+B+C = ag +bg + cg
= (a+b+c) g
= ...I guess = D g

So the answer is D grams.

_{N.B.:
I'm a certified math failure.}