1. Read all the sentences in a loud voice.

2. Read the whole sentences loudly.
3. Read the whole sentence aloud.
[Are they all correct? Do we have to use 'sentence' or 'sentences' in #2 and #3?]

1 is gramatically correct, though it would usually be "Read the sentences in a loud voice". 3 is also correct just the way it is.

2. "Read the whole sentence loudly."

Nice job, Matt.

All three sentences are correct, but there are slight differences in meaning and usage.

1. "Read all the sentences in a loud voice." - This sentence is asking you to read each individual sentence in a loud voice. It implies that there are multiple sentences, and you should read each one separately.

2. "Read the whole sentences loudly." - This sentence is incorrect because it uses the plural form of "sentences" but refers to them as a whole. To make it correct, you can say, "Read the whole sentence loudly." - This means you should read one complete sentence in a loud voice.

3. "Read the whole sentence aloud." - This sentence is correct and is asking you to read a single complete sentence out loud. It emphasizes the entire sentence, rather than separate sentences.

So, to summarize:
- Use "sentences" when referring to multiple individual sentences.
- Use "sentence" when referring to a single complete sentence.
- Use "aloud" or "in a loud voice" to indicate speaking loudly.