You can definitely ferment a 3 gallon batch in a 5 gallon bucket without any problems. In fact, it is quite common for homebrewers and meadmakers to use larger fermentation vessels to accommodate smaller batches.
One of the main reasons for using a larger bucket is to provide enough headspace for the fermentation process. During fermentation, the yeast consumes the sugars in the mead and produces carbon dioxide as a byproduct. This CO2 creates a protective layer on top of the mead, preventing oxygen from coming into contact with it and potentially causing oxidation or spoilage. By having extra space in the bucket, you ensure that the CO2 can build up without pushing the mead out of the vessel.
In addition, using a larger bucket also allows you to easily add any additional ingredients or perform any necessary stirring or mixing during the fermentation process. It provides more room for the mead to expand and bubble, reducing the risk of overflow or messy spills. This is especially important during the initial fermentation phase when the yeast is most active and vigorous.
Personally, I have fermented many small batches of mead in larger fermentation vessels without any issues. I find it much easier to handle and monitor the fermentation process in a larger bucket compared to a smaller container. It provides more room for me to take gravity readings, add nutrients or other adjuncts, and perform any necessary racking or transfers.
To further ensure a successful fermentation, it is recommended to use an airlock or blow-off tube to allow the CO2 to escape while preventing any potential contaminants from entering the bucket. This will help maintain a clean and stable environment for the yeast to do its job.
Fermenting a 3 gallon batch in a 5 gallon bucket is perfectly fine and even beneficial. The extra headspace allows for proper carbon dioxide buildup and provides room for any additional ingredients or necessary actions during fermentation. It is a common practice among meadmakers and homebrewers alike, and I have personally had great success using this method.