In your special case you can do without a regex and instead use substr and length:

while (<DATA>) {
    chomp;
    print if substr($_,0,1) eq 'R' && length == 3;
}
 
__DATA__
raa
Raa
Raab
[download]

Update: Rereading your question, my solution only applies if you read 'letter' as 'any character'.

-- Hofmator