Although it was entirely possible to control sends in Ardour 2.X, the process of doing so was a bit cumbersome. In Ardour 3, we've done three notable things to make using sends a breeze.
Inline send level controls in the redirect list
This can be seen in this image of the new redirect list, which includes a send level control inline. It is controllable in exactly the same way as other faders in Ardour (including being MIDI-learnable if you wish).
Editing send parameters in the associated mixer strip
In Ardour 2, "editing" a send meant double-clicking on its name, which would create a new dialog that allowed control of the send's I/O, level, panning etc. In Ardour 3, that same double click instead flips the mixer strip itself into displaying the state of the send. A light green highlight around the fader alerts you to the fact that you are looking at a send and not the settings for the track/bus as a whole:
To revert back to using the strip for the track/bus, just click on its name at the top of the strip.
One click to display all send levels to a given bus
In the mixer strip of an aux bus, there is now a button marked "Sends". Clicking this bus will find all tracks/busses with sends to this bus, and switch their mixer strips so that they display the relevant send's settings (as just described above). In addition, right clicking on this button will show a menu of operations that can be applied to the sends that deliver to this bus:
The first four operations create new sends to this bus, from either all tracks or currently selected tracks. The final 3 alter the levels of all existing sends to this bus. The first ("Copy track gains to sends") will make all the sends use the same gain level as the track they send from. This is an easy way to get a baseline monitor mix that will reproduce the main mix. The last two will either mute all the sends to this bus by reducing their levels or set them all to unity gain.