Is ‘sum’ the right word for a problem in mathematics?

Saying, “I solved three sums yesterday” is a uniquely Indian usage which is technically wrong.

Sum means addition. Imagine saying, “I solved three additions yesterday”! That would sound strange.

The right word is ‘problem’ or even ‘question’. Thus, you can say –
a) I solved three mathematics problems today.
b) I solved three mathematics questions yesterday.

Now, if you solved three problems dealing with ‘sum’ (or addition), you would say – I solved three problems in addition yesterday.

Remember, sum means add not problem or question.