When using hlssink3 and hlscmafsink elements, it's now possible to track new fragments being added by listening for the hls-segment-added message:
Got message #67 from element "hlscmafsink0" (element): hls-segment-added, location=(string)segment00000.m4s, running-time=(guint64)0, duration=(guint64)3000000000;
Got message #71 from element "hlscmafsink0" (element): hls-segment-added, location=(string)segment00001.m4s, running-time=(guint64)3000000000, duration=(guint64)3000000000;
Got message #74 from element "hlscmafsink0" (element): hls-segment-added, location=(string)segment00002.m4s, running-time=(guint64)6000000000, duration=(guint64)3000000000;
This is similar to how you would listen for splitmuxsink-fragment-closed when using the older hlssink2.