If you are trying to write different data to different channels, you should probably use multiple applications in your TTN account. then you can use a different decoder.
If you are just copying the same data to three channels, you could also set up an analysis in ThingSpeak that copies the data, and trigger it with timeControl or React. If you are copying with based on the data source, you can build the conditional into the MATLAB analysis that copies the code.
If you have a free field open, you could write differentiating information there.
field3=“this post came from sensor1”
The integration can tell the difference between channels by the channel ID and authentication key you provide, not by the input data.