When mendel crossed two hybrid plants for stem height (tt), what results did he always get?

When you cross two hybrids, the results always come in a 3:1 ratio as follows:

3 is the inheritance of at least 1 dominant allele
1 is the inheritance of two recessive alleles

Punnet Squares are very useful!
     T     t  
T | TT | Tt |
t  | Tt | tt  |

Three out of 4 boxes have offspring that express the dominant trait (tall plants) and 1 out of 4 boxes expresses only the recessive trait (short plants).

Hope this helps! :)