Q. Can I use the same storage account for all the VMs in an availability set?
A. While technically you can use the same storage account for VMs in an Availability Set, it is better to use different storage accounts for each VM in the Availability Set.
The goal of the Availability Set is to split your VMs in the Availability Set over 2 (ASM) or 3 (ARM) fault domains to ensure in the event of a failure of a rack, not all instances of a service are affected and during planned maintenance of Azure only one update domain at a time are impacted (by default 5 update domains).
Obviously, storage also has to be available for VMs to work and while Azure Storage is very resilient with 3 copies of the data in the storage stamp (and potentially another region with geo-resiliency enabled) it is still better to distribute the storage for your VMs in an Availability Set over multiple storage accounts to reduce the possibility of a storage problem impacting all the VMs.
