Password issue on iPhone 6s saying password is too short
The minimum passcode length for custom numeric passcodes is four characters. The same applies for custom alphanumeric passcodes.

I've also tried this on an iPhone 6s running iOS 10.3.2.

If you want a passcode that's easier to type (which I don't recommend) you may want to use repeating numbers (e.g. 1122 or 2255).