Hi
 
I am recording my drum track at the start of my first project with X3. The lanes are working differently to how they did in my previous version. 
 
This is a midi track. I am recording my drum track. The first take is my basic Kick and Snare. I am now overdubbing the fills. Later I shall add the hi hats and then the cymbals. These will all be on separate lanes. However, I have just recorded my first fill (a snare hit and two tom hits). I can now only hear either that lane or my initial lane which has Kicks at the same time as my new fill. Why can't I hear both clips? I recorded in Sound on Sound mode. I am not comping here. I want to finish up with four lanes in my drum track. One for Kick and snare, one for toms, one for hats and one for cymbals. Obviously I want to hear them all simultaneously. What am I missing here? There's obviously some new option I have to select that I don't know about. Can't find it. Help please!

Clicking with the Comp tool (Smart tool mode in the lower half of the clip) will make that clip active and mute all others. Select all the muted clips, and unmute with K accelerator key. Note that X3 no longer puts the mute symbol on muted clips, it just grays them.

I see. So what you are saying is that if I want to select a clip for any reason I need to be careful not to click in the lower half of the clip otherwise I will mute the others?
 

Exactly. And if you drag in the lower half, you'll also get the auto-splitting behavior of the Comp tool. If this happens inadvertently, you can heal the splits by sweeping through the whole clip, starting outside its end boundary, with the Comp tool.

What Dave said.    When comping it's important to watch the cursor since it will change depending on the hot-spot you're currently over.
 

And for that reason if you just want to select a clip or clips I recommend right-click dragging on the clip(s).