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
One option is to create a custom connector that consumes a service web created on Dynamics AX 2012.
I read your posts and the approach looks god. Note that the above solution Mobile Apps Connector does all that dynamically (custom connectors, metadata, authentication etc.) based on configurations in AX.
Read more at www.fauapps.com