First, the variables would be specified as "a,b,c" (without the quotes).
Second, "Option Strict" is a compiler option that's there to help you from introducing logic errors.
For example, set the type for variable "a" to Boolean and set it to true. Set the type for variable "b" to Int32 and set it to 3. Open the script and add "Option Strict Off" at the top and then run the task. With checking off it will not only run but give you an incorrect result.
To make this work, explicitly convert the variables before adding them.
CInt(DTS.Variables("a").value) + CInt(DTS.Variables("b").value)