There are several ways. A simple solution is to add parenthesis:
strz = ("This is a line" +
str1 +
"This is line 2" +
str2 +
"This is line 3")
If you want each “line” on a separate line you can add newline characters:
strz = ("This is a line\n" +
str1 + "\n" +
"This is line 2\n" +
str2 + "\n" +
"This is line 3\n")