Is it suitable for an MS student to ask teaching faculty for a recommendation letter for PhD applications?
(I'm assuming that the application form says you need three letters, but does not specify more). 
As you already have two letters, I think a third one from teaching faculty is completely okay. Most PhD programms do not expect you to have collaborated with three different researchers before admission.

Make sure they mention your work ethic in their letter (that is, if your work ethic is good), about special knowledge you have because of teaching this course (if any) etc.
If they can say those things, this is more significant as a letter from faculty you didn't interact with.

However, if you also did serious research with the undergraduate faculty and you have a good relationsship with them, consider asking them.