Overlapping CIDR ranges for subnets

I could see three subnets having ip ranges ((172.31.0.0/20) | ap-southeast-1c, (172.31.16.0/20) | ap-southeast-1b, (172.31.32.0/20) | ap-southeast-1a). Since these have overlapping CIDR ranges, I think it's not the practice when defining CIDR ranges since connections between subnets are an issue then. Please explain.