Mixing is done by addition (summation). Analog mixers are actually built around summing amplifiers.
Adjust the relative volume of the individual tracks until it sounds right. Audacity itself uses floating point, so there is no upper limit and audio data won’t actually be clipped until you save the audio file. (You may have to reduce the master playback volume so that you don’t drive your DAC into clipping.)
After mixing (but before saving) run the Amplify effect and bring the peaks down to 0dB. Now, when you save your file it won’t clip.
There is no clipping before the mix and render with channel one. Audacity somehow brings the amplitude way up. It should not. Over the past few years I have done hundreds of mix and renders and it is the first time I am running across this problem.
The issue doesn’t make sense, unless you have the gain slider turned up on the first track, or unless the other tracks have content where the length of the first channel is. What are the dashes in your drawing?
The way I understood the illustration, the dashes are silent stretches and should not contribute to the show. When exported to a single sound file, the Project should produce three short performances one right after the other. We assume they were recorded at roughly the same volume.
I do have a question as to where you got the silences. Were they just a live microphone with you off quietly making fresh coffee in the kitchen, or did you make the silences using trickery?
And yes, moving any of the sliders to the left of the tracks may create a result you weren’t expecting. It’s possible to bump those by accident and what you think you told Audacity to do and what you actually told it are different.