FCI with Alwayson Availability Group (Asymmetric Storage) I am trying to setup a demo environment where I have a windows cluster with 3 nodes.
Node 1 will be standalone sql server and will be the primary replica. 
Node 2 and 3 will be in a Failover cluster instance(sqlclu1) and that will be the async secondary replica.

I have installed sql server as a clustered instance (Node 2 and 3) and
has a shared storage(E: drive) which is presented only to node 2 and 3 and which contains sql data files(by which I mean, the possible owner for this disk is only node 2 and 3 and also I have not connected the disk to Node 1).

I am stuck at the next step where I am trying to install a standalone sql server on node 1 for being the primary replica.  The issue is when I try to add a disk with the same name (E:) it is not letting me do that.  E:drive is just not available in the list when I create a new volume.

I am guessing I have stuffed up something and am wondering how Asymmetric Storage needs to be configured so that I can create disk with the same drive letter.

As per https://msdn.microsoft.com/en-us/library/jj215886.aspx


  Asymmetric Storage Two FCIs, one at each site on a single multi-site
  WSFC, introduce considerations around how shared storage is handled.
  Each FCI has its own shared storage. The nodes at the primary site
  share storage among themselves to form a shared-storage FCI, and the
  nodes at the DR site share storage among themselves to form another
  shared-storage FCI. The storage on the primary site is not visible to
  the nodes on the disaster recovery site and vice versa. This
  arrangement of storage, where a cluster disk is shared between a
  subset of nodes within a WSFC, is referred to as asymmetric storage.
  Before the asymmetric storage capability, shared storage needed to be
  visible to all the nodes in the WSFC (symmetric storage).  This
  Windows Server enhancement is the key piece of functionality that
  enables the FCI + AG solution architecture discussed in this white
  paper. By enabling this functionality, you can combine the shared
  storage solution (FCI) with the non-shared storage solution
  (availability groups), in a single HA + DR solution. Consequently,
  this enhancement also enables you to use identical drive letters for
  shared disk resources across data centers.


Using sql server 2014 and windows 2012R2
Can you try adding it under another drive letter, and then changing the drive letter via Disk Management. It's what was suggested by Microsoft during the CTPs.