It's been my experience that a 2:1 (Rear : Front) replacement is my norm on Triumph Rocket III's (2019s and older).
Not sure if this is true for R3s but I suspect similar weights, similar rubber (AVON / Metzeler), and riding situations.
CAVEAT: This is my experience, your situation may be completely different. It's whatever you are comfortable with and what is required for the safe operation of your bike!
My $0.02