Read performance when indexing one column vs indexing multiple columns
The index is a guide to the main table.  So your first query will use the Index on column A to identify which rows in the table have the data it wants, then does a key lookup to get the rest of the data and do further filtering if necessary.  

So your second query will also use Index A and then do a key lookup and then do further filtering as required.

But you can have just a single index on A,B and C and your first query will still use that index (providing that A is the first column indexed). 

So you can have your cake and eat it too.  I wouldn't create both indexes, I would just create the one on A,B and C.  However, If your table is narrow and only has four or five columns then I would only index on A.