You can put a post next to or through the pot and into the ground to support the vine. I have an "Ison" umbrella fashion in a 25 gallon pot.

Note that with seedlings you may well get female and male vines, and if you save just the female(s) you will get no fruit.

Gonebananas, I'm curious what the umbrella method looks like. You wouldn't have a picture, would you? I tried to google it but didn't find anything.

I think this will likely be my best option, but I wonder can I prune it back and retrain it to a trellis if I decide to move it into the ground? Also, does yours ever blow over in the wind? I planted one in gritty mix and the other in 5:1:1. I'm not sure if the 5:1:1 will be heavy enough.

I'm keeping my fingers crossed that I do get at least 1 female. I already have an Ison's planted in the back yard to serve as a pollinator. These seeds may have come from a Supreme (not 100% sure), so they should flower at the same time.

Build a permanent trellis and if you dont like those grapes, dig them up and plant a different variety, unless you dont like grapes at all, then throw them out now and try to grow a different fruit...

Derek, a permanent trellis for 3 grapes isn't practical for me at the time due to cost and space. I only have space to grow the best of the 3 seedlings and don't want to invest in the infrastructure to grow 3 of them.

I don't have a photo, sorry.

The umbrella style looks a mess with muscadines because they are so strong growing, even in a pot. I have to trim the growing canes to keep them off the ground even after dormant trimming back to the top.

If you start the umbrella at the height you eventually want or slightly lower (but height above the pot soil, not the ground) you will have no trouble starting new or selected laterals when you plant to the trellis.