problem while adding Custom Libraries to CI3

Well, neither CI_Loader::library(), nor any method in its stack trace call the load_class() function. And also, line 192 in system/core/Common.php for CI3 is this:

Code:

is_loaded($class);

... which couldn't possibly trigger the error message that you originally reported.

So, you most certainly have made modifications to the system/ directory, even if you didn't put your custom libraries in there. And that doesn't exclude the possibility of you having a MY_Loader override or one of your custom libs calling load_class() itself.