MongoDB 3.4.0-rc3 is released`, marking the culmination of the `3.3.x` development series, includes several key improvements and bug fixes, including configurable connection pools size for `mongos`, limiting total memory utilization for bulk index builds, additional tests for views on sharded collections, and updates to Mozilla Javascript ESR version `45.4.0`. The release also addresses performance regressions and issues with tools like `mongoreplay`.