It seem there is not an standar connector to AX 2012 R3 on premise. It's in your roadmap?
In case not, which options the standar allows?
@SameerCh can comment on AX 2012 R3 on premise connector.
Maybe a third party solution could be an option?
Mobile Apps Connector from Fau Applications implements AX 2012 (and AX 2009) integration to the Power Platform. Read more at www.fauapps.com