E-3 is too far away, means the radar of it can't pick up the small cruise missiles as arrive. only close to the boarded, but even Liviv is out of the detection area.PAC-3 do have Link-16 capability which would mean they can receive data from E-3's in the area, though, which would allow the batteries to operate without turning on their own radars.
Of course, that will depend on whether they do receive PAC-3 which is unlikely given that these are far more essential for the US strategy in the Pacific, so it is more likely they receive PAC-2's or even earlier variants.
And if Russia wish they can easly jam all E-3 from Belorus/Russian territory. Basic math, if the radar has to detect something on 300 km then a jammer on 1200 km can mask the target.