Exchange 2010 SP3 does not introduce any changes to the EWS schema or WSDL files. This means that you can use the RequestServerVersion element value of Exchange2010_SP2 to target Exchange 2010 SP3. That said, two changes in Exchange 2010 SP3 do affect EWS clients.
First, the throttling budget for Exchange impersonation scenarios uses a clone of the EWSMaxConcurrency throttling budget. This means that a user has a separate throttling budget than an account that is impersonating that user.. For more information, read the Throttling considerations for applications that use EWS impersonation section of the EWS Throttling in Exchange 2013 article.
The second change involves Exchange 2010 and Exchange 2013 coexistence. In versions of Exchange earlier than Exchange 2010 SP3, the Client Access server version has to match the Mailbox server version. Starting with Exchange 2010 SP3, a client that sends EWS requests to an Exchange 2010 Client Access server can target an Exchange 2013 mailbox.