Too wet? That's a tantalising contention. Can you explain a little? If it's too wet for cabbage it's very likely to be too wet for most vegetables and many ornamentals. From your questions about your mystery weed I thought you were going to have an in ground garden? How is that going to work?

I would not grow cabbage in containers but a gallon per cabbage would work imo providing you feed assiduously.

I have grown 3 heads in an Earthbox one time. EB is basically a 10 gallon container but is self watering too. The 3 heads did ok, not as large as normal - they were Early Jersey Wakefield as I recall - but it did get crowded enough that the 2 outside heads layed over. Mammouth get much larger than EJW does so I would probably have only done 2 heads of it in the EB.

A 3 gallon pot is too small to grow most anything except some small herbs IMO but I suppose you could try 1 head in it.

Dave

YES having a in-ground bug these cabbage cant wait another week, IMO, to wet to till ground right now to start garden.

might be better to just dig a hole and put them in.