One password across all devices is do-able. You just have to take a moment with each device to set it up.
Your iPhone and iPad each has a passcode to open the device. Passcodes are independent of each other. You may set the same passcode on each device, but that must be done separately on each device. Changing the passcode on one device does not change it on any other.
Set a passcode on iPhone - Apple Support
Set a passcode on iPad - Apple Support
The Mac computer has a login password that is required to login and use the computer after you turn it on. You may set the login password on the Mac to be the same code you use on your iPhone and/or iPad if you like, but that must be done on the computer. And again, changing one device password or passcode does not change any other. Change the login password on Mac - Apple Support
Lastly, your Apple Account (Apple ID), also has its own password. Just like the others, this password is independent of any other device passcode or login password. And just like like the others you can set it to be the same code if you wish. Change your Apple Account password - Apple Support
If you use any device to change your Apple Account password, you must notify each device of that change, usually by signing the device out of iCloud in the device settings and then signing back in using the new password.
Syncing your Contacts from one device to another can be done without iCloud by simply connecting the iPhone and/or iPad to the computer with the charging cable it shipped with and using a Finder window to configure what gets transferred. Sync your iPhone, iPad, or iPod using your computer - Apple Support
Normally you don’t use your Apple Account password to get into any of your devices. But should you encounter a problem on one, there are steps to take that will allow you to reset your device's password or passcode using your Apple Account.
