gerunds - Should we always use "verb+ ing" structure after the verb "start"?

As far as I know, after the verb "Start" we should use verb + ing structure. For example:

• I have to start doing the exercises.

What if we use the verb "start" in present continuous?Is is correct to say/write :

• I am starting doing the exercises.

Or we should change it in the following way:

• I am starting to do the exercises.