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I assume you want the output to be 3 topics with the messages being published at the same time and that you do not care about the exact timestamp of the message that you are getting.

For this, you can make a node that subscribes to each of the three topics and populates a buffer for each of them. It should have a timer that runs at the required rate and takes the top message from each buffer, rewrites the timestamp and writes it into a rosbag.

Alternatively, you could repeat the messages that are at a lower rate if you the final rate to be higher than 20 Hz (the lowest frequency)