Here's a grammar quiz on solely third and mixed conditional clauses together. They have very close uses and forms. After a grammar presentation, this PDF worksheet is excellent to consolidate both clauses in a meaningful context. All questions focus on the main differences between the two clauses and they are explained well in the feedback notes.
What should be considered as a signal to choose between Third and Mixed conditional clauses?
I think the completion of both clauses in the past is the most critical aspect of choosing third conditional forms. Both parts of the third conditionals refer to the past, which contradicts with mixed conditionals as at least one part should refer to present in mixed types. Also, inferring the counterfactual present possibility aspect in mixed clauses will help students deciding on the correct option in the grammar quiz.
There's also an online version of this material.