I have my singing class at 3 and need to confirm with my instructor. Is this the right way to ask the question?
I think the simple idiomatic "on" might have started out as short for "on the schedule". At any rate, Merriam-Webster lists definition #4 of "on" (as an adjective) this way:
Their example is, "has nothing on for tonight"
So your sentence sounds fine in English:
"Is the class still on at 3?"
In writing, it looks a tiny bit tricky at first because of the two little words "on" and "at" right next to each other, but there is nothing wrong with the sentence. It is grammatical, idiomatic, natural, and easy to understand -- especially in spoken English. And, if the reader is paying attention, it is also no problem in writing.
You could also ask,
"Are we still on for class at 3?"
"Is our/the 3 o'clock class still on?"
All 3 sentences are good.