Can I paint upholstered furniture without fabric medium ?

The purpose of the fabric medium is to keep the painted fabric from getting hard, stiff, & scratchy. Thinning the paint will not do that. If you don't want to use fabric medium, then you should buy fabric paint. The fabric medium will save you a lot of money though. There is a diy fabric medium recipe out there but every one that I found had several cautions and it appears that it's not used for furniture. The biggest and most important caution was not to use too much glycerin because it can break down the paint. The issue with the paint is that every manufacturer uses a different formula and you don't know what's in it or how the glycerin will react. BTW, you can buy fabric medium at several craft stores and all of them have great coupons weekly and sometimes daily. I had one yesterday for 60% off a non-sale item and there was another one in my email today. This link just shows the recipe, but I'd say use with caution. Wishing you the best.

https://makeup.wonderhowto.com/how-to/make-homemade-mixing-medium-222939/