Gallons and Liters

1. You could always use a 3L as a primary as well, if you don't lose a liter in transit. Or make several gallons in a larger container (plastic buckets are cheap and often come with spigots) and then transfer to several 3L containers for secondary/bulk aging. I usually top up my gallons after a few days to a week--once the activity is slower and there are less bubbles. This way I usually end up with about 110-120 fl oz of mead after transfer (128 fl oz in a gallon). Of course, you don't have to top up and/or you can drink the "extra".

2. If your mead is not actively fermenting (which is when it produces lots of CO2), it should be kept in a container such that head space (air above the liquid) is minimized. This goes for most secondary fermentations, bulk aging, bottling, etc. If you have a way to inject CO2, N2, Ar, etc into headspace (to drive off oxygen), this is less of an issue. Basically, too much oxygen is bad for mead that is done fermenting (and too much can be a pretty small amount). But for primary fermentations, don't fill to the top! You'll end up with some sticky explosives and fragrant ceiling decorations.

Use a gallon primary, then rack to the 3L carboy when you rack off the yeast and stuff in the bottom, and then you will have a little bit left over for topping-up later (or for sampling :drunken_smilie: ).