Look at the power level of each hero. A fully developed talent grid will add about 100 power to the hero. That is technically enough to bump a 4-star at 70+20 up to the power level of a 5-star at 80+0, but in reality the 4-star still falls a hair short.
3-stars however remain hosed no matter how many emblems you give them. Notice how their power starts off about 200 below a 4-star. By maxing a 3-star’s talent grid you bridge about half of the gap. Meaning it will then be about as powerful as a 4-star at 3-60 but my money says that it will still fall just a little bit short based one the shortcoming of 4-star to 5-star heroes.