I believe these numbers represent the thread, client PID, and session ID, respectively. You can look at the logconfig file to see how the message lines are being formed:
$ grep Conv /opt/sas/viya/config/etc/cas/default/logconfig.xml
<param name="ConversionPattern" value="%d %-5p [%t] %X{Cas.ClientPid|MAIN} %X{Cas.ClientUser|NoUser} %X{Cas.SessionIndex|MAIN} [%F:%L] - %m"/>
<param name="ConversionPattern" value="%d %-5p %X{Cas.ClientPid|MAIN} %X{Cas.ClientUser|NoUser} %X{Cas.SessionIndex|MAIN} - %m"/>
<param name="ConversionPattern" value="%-5p [%t] %X{Cas.ClientPid|MAIN} %X{Cas.ClientUser|NoUser} %X{Cas.SessionIndex|MAIN} [%F:%L] - %m"/>
Syntax for a Pattern Layout
https://go.documentation.sas.com/doc/en/pgmsascdc/v_027/logug/n0t35nn3zod1enn1x72nmd9bcbu8.htm
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Greg Wootton | Principal Systems Technical Support Engineer