Step 1: First blank.
The sentence begins:
\[
__________ you have a doubt, why not go and verify?
\]
Since this is a conditional clause, the correct conjunction is
if, not “as.”
\Rightarrow “If you have a doubt...” is correct.
Step 2: Second blank.
Phrase:
\[
\text{I shall be waiting in the shade __________ this banyan tree...}
\]
The correct preposition is
of. We commonly say “the shade of a tree.”
\Rightarrow “in the shade of this banyan tree...” is correct.
Step 3: Third blank.
Phrase:
\[
\text{...till you come back __________ me.}
\]
Here, the preposition should be
to, because we say “come back to me.”
Step 4: Verify sentence.
Final sentence:
\[
\text{If you have a doubt, why not go and verify? I shall be waiting in the shade of this banyan tree till you come back to me.}
\]
This is grammatically and idiomatically correct.
\[
\boxed{\text{if; of; to}}
\]