You can create directly multiline strings with the caret (one empty line is required).
setlocal EnableDelayedExpansion
set multiLine=This is a ^
multiline text^
line3
echo !multiLine!
Or you can create first a newline character.
setlocal EnableDelayedExpansion
set LF=^
rem Two empty lines are required
set multiLine=This is a!LF!multiline text!LF!line3
echo !multiLine!
An explanation how this works can be found at Explain how dos-batch newline variable hack works