AWS Cloudformation conditions

I would like to have an IF-ELSE statement to assign subnets.

If 1 AZ is being used, assign privatesubnet1 to subnets

If 2 AZ is being used, assign privatesubnet1 and privatesubnet2 to subnets

If 3 AZ is being used, assign privatesubnet1, privatesubnet2 and privatesubnet3 to subnets

The IF function handles 1 and 2 AZ well, but I don’t know how to handle 3 AZ

Suggestions are welcome.

Conditions:
  SingleZone:
    !Equals [!Ref NumberOfZones, 1]
  TwoZones:
    !Equals [!Ref NumberOfZones, 2]
  ThreeZones:
    !Equals [!Ref NumberOfZones, 3]
Resources:
  Properties:
    Subnets:
      - !If [TwoZones, !Join [',', [!Ref PrivateSubnet1, !Ref PrivateSubnet2]], !Ref PrivateSubnet1]
      #How to handle ThreeZones