F = [repmat(str1, 1, 3), repmat(str2, 1, 6), repmat(str3, 1, 12)];
In the special case where your items are all only single characters,
F = repelem([str1, str2, str3], [3 6 12]);
Both of these would produce 'aaabbbbbbcccccccccccc' .
But possibly you want
F = [repmat({str1}, 1, 3), repmat({str2}, 1, 6), repmat({str3}, 1, 12)];
or
F = repelem([{str1}, {str2}, {str3}], [3 6 12])'
if your desired answer is
'a' 'a' 'a' 'b' 'b' 'b' 'b' 'b' 'b' 'c' 'c' 'c' 'c' 'c' 'c' 'c' 'c' 'c' 'c' 'c' 'c'
If you have R2016b or R2017a and have constructed your items as string objects, like (R2017a or later)
str1 = "a"; str2 = "b"; str3 = "c";
then you can use
F = repelem([str1, str2, str3], [3 6 12]);
if your desired output is
"a" "a" "a" "b" "b" "b" "b" "b" "b" "c" "c" "c" "c" "c" "c" "c" "c" "c" "c" "c" "c"
and you can use
F = strjoin( repelem([str1, str2, str3], [3 6 12]), '' );
if your desired output is "aaabbbbbbcccccccccccc"
