Step 1: Identify the time reference.
The phrase “at three o'clock tomorrow” refers to a specific time in the future.
Step 2: Recall the correct tense.
To describe an action that will be in progress at a particular time in the future, we use the
future continuous tense:
\[
\text{will be + verb-ing}
\]
Step 3: Analyse the options.
Working — incorrect, auxiliary verb is missing.
Will be working — correct future continuous form.
I'll be working — contraction of “I will be working”, also correct.
Both Will be working and I'll be working — correct.
Step 4: Choose the best answer.
Since both (B) and (C) are grammatically correct,
Hence, the correct answer is
\[
\boxed{\text{Both Will be working and I'll be working}}
\]