1. "á and à" are the same, but just "á" does not exist. When using just the character "a", the correct is "à". The pronunciation is practically the same as "o" in "ouch".
2. "ã and a" are the same and are practically the same as "un" in "under". When used as a letter, "a" has the same pronunciation as "à". Again, just "ã" does not exist.
3. "â" is the same as "ã". Again, just â" does not exist.
This is the general pronunciation. It all depends on the word in question. Especially the character "a".