Microsoft released RU12 for Exchange Server 2007 SP3.
You can find this update here
As always for Exchange 2007 you can download 32bit or 64bit version of this update depending on your system version.
When you install RU12 for Exchange 2007 SP3 you will get exchange version number: 8.3.342.4
More informations about this update you can find here: KB2903911
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