I have tried to begin a return for my mother's estate and although I selected the custom forms button I cannot begin a T3 form. Does TT2017 support T3 Estate returns?

Unfortunately, there is no version of TurboTax that can be used to prepare a T3. However, Intuit offers another product called Profile that does have the ability to do T3 returns. http://profile.intuit.ca/professional-tax-software/profile-onepay.jsp