It's one thing for the highly competitive environment of "rate table" advertising (where lenders compete and you get confused) to be reporting mortgage rates  approaching the mid 3's. In fact, in that arena, rates have been in the 3% range for quite some time. Whether or not you'd qualify or even be interested in the specific scenario that is conducive to such rates is another story.

It's an entirely different thing for me to be telling you that rates are now easily into the high 3's, because I'm talking about the most prevalently-quoted conventional 30yr fixed rates for the average top-tier scenario across all lenders.  3.875% is now a given at almost any well-priced lender, provided you have a high credit score and a decent amount of equity.  3.75% certainly isn't out of the question for the best scenarios, and the aggressive lenders have no issues quoting 3.625% today for perfectly-aligned stars.

With all of the above having been said, please keep in mind that just a few "hits" to your scenario (slightly lower credit, lower downpayment, less aggressive lender) and you shouldn't be surprised to see a "4%" as the first number in your 30yr fixed rate quote instead of a "3%."  

As for the motivation, today's gains were primarily a factor of news relating to North Korean missiles flying over Japan.  It was a bigger-than-normal improvement in the recent context, but only a medium-big move in general.  It was simply made more noticeable by the fact that rates were already at 2017's lows yesterday.

Should you lock now?  Great question!  No one will ever be able to predict that future for you, but here are a few thoughts.  As recent improvements have persisted, a pull-back becomes increasingly possible.  Additionally, there's never any harm in locking the lowest rates in nearly 11 months.  On the other hand, lenders have been slow to adjust rate sheets to reflect bond market movement, so if you're exceptionally risk-tolerant, you could likely escape with minimal damage if markets happened to move against you.