In existing versions of Ardour there are two "redirect lists" - places where you can add plugins, sends and returns to the signal flow (we call them "redirects" because they all "redirect" the signal flow from what it would otherwise be). Ardour 2.X shows one before/above the fader and one after/below it.
In Ardour 3.0, these two lists have been unified, and show the position of the fader explicitly. You can now move plugins around freely from pre- to post-fader positions and back again. More interestingly you can also choose to make the meter position visible (switch meter mode to "custom") and then position the meter processor wherever you want. This allows you to do metering between plugins, rather than just at the input, fader or output stages.
The redirect list above shows a signal flow that will pass through Giant Flange plugin (pre-fader), then an aux send, then the fader and then a Gate plugin (post-fader). The meter position for this track was not set to manual, so the meter position is not shown. If we switch it to "custom", we would see:
You can now see the meter unit at the end - it can drag-n-dropped to any position in the list to give precise metering at that point in the signal flow.
Note also that redirect active status is now shown by an explicit check box on the left, where you can click to enable/disable the plugin/send/return, rather than by using colors and naming as was done in Ardour 2.X. Finally, as mentioned in the entry on aux send control, notice that there is now an inline control for the send gain level in the redirect list itself, avoiding the need to use a different window for the most basic send operation: changing the level.