My second else: statement is throwing up a syntax error, any idea why?

This is code for checking for sideswap numbers:

```
number1 = int(input("Enter a number between 0 and 9")) 
number2 = int(input("Enter a number between 0 and 9"))
number3 = int(input("Enter a number between 0 and 9"))

X = number1 + number2 + number3

while True:
        if X%3 !=0:
            print("Not a valid sideswap number")
            number1 = int(input("Enter a number between 0 and 9"))

            number2 = int(input("Enter a number between 0 and 9"))

            number3 = int(input("Enter a number between 0 and 9"))

        else:
            if number2 - number3 == 1:
                print("Not a valid sideswap number")
                number1 = int(input("Enter a number between 0 and 9"))

                number2 = int(input("Enter a number between 0 and 9"))

                number3 = int(input("Enter a number between 0 and 9"))

        else:           
            if number3 - number1 == 1:
                print("Not a valid sideswap number")
                number1 = int(input("Enter a number between 0 and 9"))

                number2 = int(input("Enter a number between 0 and 9"))

                number3 = int(input("Enter a number between 0 and 9"))

        else:
            if number1 - number2 == 1:
                print("Not a valid sideswap number")
                number1 = int(input("Enter a number between 0 and 9"))

                number2 = int(input("Enter a number between 0 and 9"))

                number3 = int(input("Enter a number between 0 and 9"))

```