Select the phrase that will make the following sentence grammatically correct. Three days from now, the horses ___________.

In this sentence, 'Three days from now' indicates a future timeframe, so the correct verb tense to use is the future tense. The phrase 'will perform in a show' is the best choice to make the sentence grammatically correct because it aligns with the future action of the horses. Choice A ('performing in a show') uses the present participle and is incorrect in this context. Choice B ('performed in a show') uses the past tense, which is not suitable for a future event. Choice D ('have performed in a show') uses the present perfect tense, which is also not appropriate for a future action. Therefore, the correct answer is C.