You don't want to do that. If it's in secondary, very little fermentation is taking place and you shouldn't have headspace in secondary. You could just wait to get a 3 gallon carboy, or you could very carefully without splashing rack into the larger carboy, clean the 3 gallon, sanitize it and rack it back without disturbing it much. I would only do that if it desperately needed to be racked with very thick gross lees. Otherwise, I would just wait until I had the right size carboy.
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