Shrinkrap,
I see you have the Organic Tomato Success Kit, correct?
https://www.gardeners.com/buy/organi...8587289VS.html
and it comes with 40 quarts of soil. So that would be ten gallons of volume for the four plants.
My three watermelon vines are in a Gro Pro Premium Round Fabric Pot (7 Gallons). The soil calculator
https://www.hawthornegc.com/tools/soil-calculator
says that's 32.91 quarts of soil for the pot. I've filled it almost full.
I think three watermelon vines for a pot of this size is a stretch (I couldn't bear to let any one go this summer), even if one is a "bush" (short vine) variety (Bush Sugar Baby). I'm feeding them every two weeks and watering constantly, by hand, not on a drip system. Next time I'll go for two per pot max.
Hope the yellow melons are tasty! Fully ripe with the brown tendril!