I am mushroom ^-^

First off, since the bags hold a maximum of 30 mushrooms and they all add up to 90, all the bags will have exactly 30 mushrooms each.

There are 2 ways you can think about this:

The answer is $\frac{{90\choose30} {60\choose30} {30\choose30}}{3!}$

We're choosing 30 mushrooms that we'll put into the first bag, 30 mushrooms that we'll put into the 2nd bag and 30 that will go to the 3rd one. Since the bags are identical, to eliminate the ordering of the bags we'll have to divide by $3!$ as well.

Arrange the mushrooms in a row. The amount of ways we can rearrange the distinct mushrooms in a row is $90!$ . The first 30 mushrooms will be in the first bag, the other 30 in the 2nd and the last 30 in the 3rd bag. Since the order in which the mushrooms appear in the bags doesn't matter, we'll divide this by $30!30!30!$ and since the bags are identical we'll additionally divide this by $3!$ , yielding $\frac{90!}{30!30!30!3!}$

Both ways lead to the answer $\frac{90!}{30!30!30!3!}=13267964927921872702429057742393630441856$

Wolfram Alpha can tell you this. It can also tell you that the digits of this number add up to $\boxed{186}$ .

Generating functions.