Making R3 an L1/L2 router restores the required IS-IS hierarchy and prevents the outage caused by the failed R1-to-R2 link. In IS-IS, Level 1 routers have detailed topology only inside their own area, while Level 2 routers provide the interarea backbone. Routers that connect a Level 1 area to the Level 2 backbone must operate as Level 1/Level 2 devices so they can participate in both databases and provide an exit path for area traffic. If a path between areas depends on a router that is only Level 1 or only Level 2 in the wrong location, traffic can be blackholed when a key link fails. Configuring R3 as L1/L2 gives the topology an additional valid transition point between the local area and the backbone, improving continuity after the R1-to-R2 failure. Making unrelated routers L1 or L2 does not necessarily create the required interarea attachment.
Reference topics: IS-IS Level 1, Level 2 backbone, L1/L2 routers, interarea reachability, hierarchical IGP design.