How to deal with sometimes unused arguments of function?

Question

Could you please advise me how to deal with sometimes unused arguments of function? Thank you in advance!

Function:

def foo(a, b=2, c="default"):
  print(c)
  return a + b

Usage:

arg_1 = int(input())
if arg_1 == 2:
  arg_2 = 3
  arg_3 = "using 3"
else:
  # what can I do here?
  arg_2 = *universal_value_that_is_disabling_argument
  arg_3 = *universal_value_that_is_disabling_argument
  # I know that I can use arg_2 = 2 and arg_3 = "default", 
  # but it is not really convenient when there are many arguments

foo(arg_1, b=arg_2, c=arg_3)

I understand that I can do something like this, but it is not really convenient when there are many arguments:

arg_1 = int(input())
if arg_1 == 2:
  foo(arg_1, 3, "using 3")
else:
  foo(arg_1)