It's part of the way DOCSIS works, every 30 seconds or so the Hub sends what's called a Ranging Request message back to the CMTS in the local headend. This message is basically a 'I'm still here message' and the CMTS should send a response back within 200ms. The response is called (not surprisingly) a Ranging Response and contains information that the Hub may need to process, slightly change timing, power levels, frequencies etc. If the Hub sends out a request and doesn't get a response back then it flags the error and resets it's clock. The CMTS needs to know regularly that the Hub is still active otherwise it won't allocate a time period, called a MAP Grant, in which it can transmit upstream data.
If the Hub misses sixteen consecutive RNG-RSP windows then it assumes it has lost connectivity and resets itself to try and regain a connection. These T3 events generally signify a problem with the upstream, it's more likely that the Hub sends the message out but the CMTS never receives it and hence doesn't sent the response.
The thing to bear in mind is that the occasional (one or two a day) is fine, the CMTS might be just too busy, there may be excessive latency on the upstream and the message doesn't get there in time; upstream latency is always an issue with DOCSIS, purely because of the way it works - but that's a whole different ballgame! You'll see the T3 events and think something has gone horribly wrong, but what isn't logged and you don't see are all of the successful RNG-REQ/RSP transactions.
You just need to watch out for excessive numbers of these occurring with increasing regularity.
Incidentally the other common log entries you may see are Sync Timing and MDD timeouts, which normally are indicative of downstream issues but again nothing to worry about if they are infrequent.
By the way, the above is somewhat over simplified but hopefully answers your question.
