One of the instruments in LMMS is the Triple Oscillator. It is composed of three audio sources such as 3 sine waves. In the default mode (MIX), the output is just the sum.
You can select the Triple Oscillator from the Instrument plugins, near the My Samples button. A list of instruments will appear and you can drag the Triple Oscillator to the Beat + Bassline Editor. You can remove the Kicker instrument from the Editor if you want.
This is the pattern selected:
By playing it, you will notice the master is clipping often as seen by the reddish colors. By exporting the wav file, you can verify the clipping in Audacity:
A solution is too decrease the time of each note. By clicking on Triple Oscillator in Beat + Bassline Editor, the Triple Oscillator setting dialog appears. Select the ENV/LFO tab, and set the hold to be zero, by changing the HOLD knob.
Now exporting the file, we see that the notes are not touching each other.