lets say your third variable is
String result
use custom code block and "add" them together:
result = hello + (string)abc;
(string)abc means, that abc you want convert to string from double ...
if you want space between, than this:
result = hello + " " + (string)abc;
if you want to add something own:
result = hello + " " + (string)abc + " myText";
if you need precision from double number, do this:
result = hello + DoubleToStr(abc, 4);
where number 4 means how much digits you need after decimal ...