hi,

for 1500 users, HA, Archive. planning tool says 3 Enterprise lync 2013 servers, should it not be 2 servers, as one server can accomodate up to 6600 users

The High Availability requirement gives you the edition needed (Enterprise) and Microsoft's recommendation is to have 3 nodes in a Front End pool due to windows fabric's quorum. 

http://technet.microsoft.com/en-us/library/gg412996.aspx

"We do not recommend deploying a Front End pool that contains only two Front End Servers. If you do ever need to deploy such a pool, follow these guidelines:

• If one of the two Front End Servers goes down, you should try to bring the failed server back up as soon as you can. Similarly, if you need to upgrade one of the two servers, bring it back online as soon as the upgrade is finished.
• If for some reason you need to bring both servers down at the same time, do the following when the downtime for the pool is finished:
  
  • The best practice is to restart both Front End Servers at the same time. 
  • If the two servers cannot be restarted at the same time, you should bring them back up in the reverse order of the order they went down.
  • If you cannot bring them back up in that order, then use the following cmdlet before bringing the pool back up:.

    Reset-CsPoolRegistrarState -ResetType QuorumLossRecovery -PoolFQDN <FQDN>

"