How to get optimized value of thetas in the equation?

There is a method “THE THETHA PROJECTION METHOD” for predicting the creep curve where creep is the tendency of a material after being subjected to high levels of stress, e.g. high temperatures, to change its form in relation to time.Creep strain (e)can be related to time(t) with following equation.
$ $ e = \theta_1 * (1 – e^{-\theta_2 t}) + \theta_3 * (e^{(t * \theta_3)} – 1) $ $

I have data of e vs t .How can we get most optimized set of theta using data of e vs t with minimized error?