Hi Asha. A lot would depend on the plant species used and its size. An Arabidopsis plant in 3 kg of soil is highly unlikely to be stressed, while a mature fig tree probably would be. It is also not a good idea to say that you want to prove that something is a stress factor. This can easily lead to a one sided approach that will indeed provide the answer that you want. It is is much safer and correct to say that you would like to test the hypothesis that 3 kg of soild mix is a stress factor. I think that you need to define your parameters better, what kind of soil mix, what are the nutrient levels of the soil, what plant species will you be using, etc.

3 kg of soil may at best give you a possible rooting volume of less than 1 L and this is certainly stressful for large plants like trees and shrubs. This sort of stress, via restricting the root growth, is commonly termed root restriction. We have a paper published about how one plant hormone, cytokinin, was affected simply by reducing rooting volume.

https://www.researchgate.net/publication/230744531_Effects_of_root_restriction_on_growth_and_associated_cytokinin_levels_in_cotton_(Gossypium_hirsutum)

Article Effects of root restriction on growth and associated cytokin...