The Huns (or possibly another similar group of people from the Asian steppes) arrive in Europe in the 3.century AD.
Huns probably weren't formed as a people in the IIIrd century.
As far as I understand, the Huns are probably the same people that were called
Xiongnu
Not exactly : they might be issued from Xiongnu, but their ethnogenesis as a distinct people went after mixing up with Iranic, Turkic and Proto-Slavic elements.
A bit like XXIth century Americans aren't XVIIth century English, but you can still point a continuity.
Note that Sarmatians were coming from Western Asian steppe as well than Eastern Europeans : Ural never was a border for anyone then. Making the distinction is a bit...well, moot.
Now, giving that Hunnic migrations may have been blostered partially by climatic changes (droughts, apparently), and that you can just make these happening randomly, you'd need geopolitical changes.
Maybe Xanbei forming a khaganate, with weaker Chinese states, propelling Tiele and therefore Proto-Huns (that lived among Sarmatians at this point*, so apparently it won't fit with your OP, but admittedly there's few odds it would be avoided by the IIIrd century).
Basically a stronger Sarmatian-German pressure on the Danube, which may not be that world-shattering, to be honest. Rome managed to deal with that, and giving the situation, I doubt that whichever people would be in charge, if they even change (although you may always bet on Proto-Huns, peoples issued from Tiele, etc.), could really modify widely the geopolitical situation.
It may even proove being a stabilizing factor, as Huns were in their time as making more immediatly agressive peoples entering in their clientele (while it may backfire, while a stronger Romania after the Crisis would be harder to deal with than during the Vth century).
*Even Late Antiquity Huns were pretty much Sarmatized, at the contrary of the Mongol-like depiction you still have from time to time