we currently have two exchange 2013 servers running on 2008r2 at our main office.  they are in a DAG, everything replicating correctly.

we are going to add a 3rd (virtual) 2013 exchange server at a remote site in the near future.  that site is being setup for DR, we have a 50 MB connection to the site.

I have been doing some research today on site resiliency, but I have not seen any examples where you had only 3 exchange servers.

can I do site resiliency  with only 3 servers?

will I have to have a 3rd site for witness server?

all we are trying to do, is to set it up so that if site 1 with the two exchange servers go down for some reason, site 2 with the single exchange server will then be available to keep email flowing throughout the company.

we currently have two exchange 2013 servers running on 2008r2 at our main office.  they are in a DAG, everything replicating correctly.

we are going to add a 3rd (virtual) 2013 exchange server at a remote site in the near future.  that site is being setup for DR, we have a 50 MB connection to the site.

I have been doing some research today on site resiliency, but I have not seen any examples where you had only 3 exchange servers.

can I do site resiliency  with only 3 servers?

will I have to have a 3rd site for witness server?

all we are trying to do, is to set it up so that if site 1 with the two exchange servers go down for some reason, site 2 with the single exchange server will then be available to keep email flowing throughout the company.

Yes, you can do that.  If you had a 3rd site for a File Share Witness, that would imply automatic failover to second Data Center with an even number of DAG members in each site.

Since you have 3 DAG nodes, you will be doing a manual switchover to the DR site if the primary is down- not an automatic one.- and with an odd number of nodes, no FSW is needed for normal operations.