I've made a map -
here, with frontier regions colored brighter than less vulnerable regions - illustrating what I personally view as the ideal division of the Roman Empire, which is technically a tripartite split though with different borders than OP's version. I wouldn't have it be divided into de facto separate empires as IRL, with different Emperors ruling over culturally distinct regions, since that's just asking for them to split and become rivals. Instead, the Emperor rules the Central part directly while delegating the West and the East to military prefects. If some sort of formal succession-by-appointment can be introduced, then these prefects should compete for the position of heir, ensuring at least one - the likely heir - would (probably) be loyal to the Emperor if the other revolts. I would not make either prefect formally superior to the other, however, since that would lessen the competition for imperial favor. It also allows for a Western prefect to be the preferred heir when experience against barbarians is more necessary, or an Eastern prefect when the Persians are the bigger threat.
I basically agree on the Western portion; Gaul, Hispania, Mauretania, and Britain. I don't view Britain as a worthwhile conquest, so I'd only want to keep the minimum necessary to have a staging area to disrupt any unification of the rest. Even that's not all too important, as long as the tin islands are secure. The Western prefect is tasked with guarding the Rhine and neutralizing any threats in Britain.
The Eastern portion I would shrink heavily to basically Aegyptus and the Diocese of the Orient. Possibly add the rest of Anatolia here instead of in the Center, if the defenses of the Orient needs more funding, but I don't think that would be necessary. The Eastern prefect is tasked with defending against Persian and Arab raids and holding the line in a full-fledged war.
The Central portion would consist of the rest, dedicated to the Danube but also to reinforcing the other two in the event of a major incursion. This also ensures Illyria is not split and so is more easily defended.
The pink indicates areas of strategic importance that are nevertheless outside the ideal defensive borders of the Roman Empire. In the north, this could lead to the establishment of an also-effective Elbe-Carpathians-Dneister border, while in the east it's mostly just denial of Mesopotamia to enemy empires based out of Persia. Whether that denial involves conquering Mesopotamia directly or simply keeping it apart from Persia is not as important, though obviously the former is preferable.
The Empire would still need a lot of other reforms beyond territorial division to survive, of course, and I wouldn't have high hopes of it succeeding with those reforms but I still like these borders nevertheless.