3xOsc is a great place to start, it's the most simple of all of them.
The first thing I do is consider range- do I want a high, medium, or low range sound? That's self-explanatory enough in the 3xOsc's user interface, the "Coarse" knobs allow you to change this. We'll make a basic pad, so pick a basic Oscillator shape based on how you want your pad to sound, and turn down the volume on the bottom two Oscillators.
The the shape of the Oscillator dictates the characteristic tone it produces. I suggest Saw, Triangle, and Sine for pretty much everything, but the Half-Square / Half-Sine sounds good sometimes too.
Next you'll want to set up your attack. Move over to the "Ins" tab. This is where most of the major sound properties come in, even though it seems very simple. Each of these envelopes change uniformly an aspect of your sound. Set the Volume Envelope to have a relatively long growth time, and a relatively long sustain. Make a basic pattern with your instrument, and work with your attack envelope until it sounds good.
So that sounds like a pad, sort of. You may want to make another 3xOsc with the same pattern, so you can have a second sound that grows later into each note, or starts out earlier. Alternatively you could enable one of the other Oscillators on your current channel and play with it's Coarse tuning, Fine tuning, and waveform until you find a uniform sound from it.
There are other envelopes you can play with, the one that would be of most use with a pad would be the "Cut" envelope. First on the bottom right of the Ins tab, there is a box that has Mod X, Mod Y, and Fast LP in it. Turn Mod Y up, with this filter it is your resonance. Now, with your pattern playing, move around Mod X and see the effect it has on the sound. You can control this in one of 3 ways.
1. The "Cut" envelope, which works exactly the same way as the Vol envelope.
2. The LFO channel which I will explain below.
3. Recording actions on the Mod X knob, which is more useful for basses where some finesse is needed.
The LFO (I don't know what it stands for) gives you wave-based control over any of the envelope properties.
The knobs stand for "Delay, Attack, Amount, and Speed," "TB" is Tempo-Based, and "Global" means that the wave is not based on individual notes. Move the amount up a little bit, and make it a Sine wave. Now, right-click on the "Speed" knob, and set it to "2 Steps." Now if you play your pattern, you should hear a cool sweeping effect on your pad.
Anyway, you can do anything with 3xOscs if you know what your are doing, have fun and give Amplitude Modulation a try on the 3rd Oscillator.