Assuming you're on Splunk 6.x, something like the following should do the trick. Also, do you care about the index as a whole or the different sourcetypes within it?
| tstats latest(_time) AS lastTime WHERE index=A OR index=B OR index=C BY host sourcetype index | eval current=now() | eval age_min=round((current-lastTime)/60,2) | rangemap field=age_min default=severe low=0-30 elevated=30-60 | stats max(current) AS "Current Time" values(sourcetype) AS Sourcetypes list(lastTime) AS "Latest Event" list(age_min) AS age_min list(range) AS Status by host | convert ctime(*Time) ctime("Latest Event") | eventstats avg(age_min) AS avg_age by host | eval avg_age=round(avg_age,2) | sort - avg_age | rename age_min AS "Communicated Minutes Ago" avg_age AS "Avg Communicated Minutes Ago"
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