The class you defined is called value class in MATLAB. The didn't directly modify the instance of the class. You need to assign it yourself
inst = inst.myfunc(arr1, arr2, arr3);
Your current code will also work, if you define your class as handle class, in that case
classdef myclass < handle
properties
a;
b;
c;
end
methods
function obj=myfunc(obj, arr1, arr2, arr3)
obj.a=arr1;
obj.b=arr2;
obj.c=arr3;
end
end
end
Read more about handle class here: https://www.mathworks.com/help/matlab/handle-classes.html