A function to take three integers and return a list of integers and alphabet-letters
PHP, 67 bytes

First of all, thanks for posting such a cool challenge! I really enjoyed solving it :) Now, here's my 67 byte solution:



&lt;?for(;$i&lt;=($p=$argv)[3];$i+=$p[2])echo$i%$p[1]?$i:chr($l++%26+97);


To run it:

php -n &lt;filename&gt; {a} {b} {c}


Or Try it online!



Here's the same solution ungolfed and with explanation comments:

$a      = $argv[1];
$b      = $argv[2];
$c      = $argv[3];
$letter = 'a';
for ($i = 0; $i &lt;= $c; $i += $b) {
    if ($i % $a) { // If $i is divisible by $a, the modulo (%) operator will return a remainder of 0, which PHP sees as a falsey value.
        echo $i;
    } else {
        $letter++;
        $letter %= 26; // Wrap $letter around to `a` after it gets to `z`.
        echo chr($letter + 97); // Use PHP's chr function to get the character. 97 is the index of `a`. http://php.net/manual/en/function.chr.php
    }
}




I did a 60 byte solution, but it doesn't wrap around :(

&lt;?for($l=a;$i&lt;=($p=$argv)[3];$i+=$p[2])echo$i%$p[1]?$i:$l++;


Try it online!