Hello!
Small note: I would rewrite your cross so the tT is written as Tt, and so the TT will be in the top left corner while the tt is in the bottom right.
The phenotype is the observable characteristics of an organism that we see. It is not the genetic makeup. In this case, we have two genes - one for tall stems and one for short. The tall stem gene is dominant, meaning that if there even is one copy of it, the plant will have a tall stem. On the other hand, the short stem gene is recessive, meaning that there must be two copies of it for it to be expressed.
Now, we see here the four genotypes: TT, Tt, Tt, and tt. There are three genotypes with at least one tall stem gene, meaning that three phenotypes will have a tall stem. There is only one genotype with two short stem genes, meaning that there will only be 1/4 of the plants with a short stem.
Therefore, our ratio becomes 3 tall stems : 1 short stem, or 3:1.
Hope this helps!