[Tee tiong huat]

Admittedly, I'm ignorant on all other details of HiRISE: issues pointing, proximity of target to Sun from MRO's perspective, camera sensor, sensitivity or exposure time may be a limiting factor here.

Edit: I have just found out that HiRISE has been used in the past to observe comets passing near Mars, notably  C/2013 A1 Siding Spring, which was imaged using HiRISE in 2014 & C/2012 S1, which was observed in 2013. In the C/2012 S1 case, imaging was done at a range of ~0.09 AU, so half as close as 3I/ATLAS will come.

https://www.uahirise.org/releases/ison.php

https://www.jpl.nasa.gov/images/pia18863...ing-comet/

Certain about imaging details. HiRISE on MRO has a 50cm mirror & 12 meters @focal lenght. Pales comparison to Hubble's 2.4m mirror, but not by that much. It's a sizeable telescope. The closest approach of 3I/Atlas to Mars @only @0.19 AU, while Earth, it will be at 1.8 AU. Means that 3I will be ~10 times closer to MRO than to HST at closest approach. HiRISE mirror is only ~5x smaller than Hubble's. So, HiRISE might be able to get some good images of the comet.