Because the guns are located in different parts of the field. They're far enough away that the required lead and travel distance of the shell from one gun will be significanly different from the shell of another.
Also theres like, what, 3 88's on a field? Sorry, but 3 '88s aren't going to throw up a 'heavy' barrage. At best, it could be considered worrisome.
And in addition to that, theres actually some shell randomization for the 88's, and they seem to require a pretty close hit to destroy or significantly damage a target. At long range (where you want to stop the bombers), that shell randomization is going to come into play.
So while that MIGHT be effective for US 90mm's, with proxy fuzes, it wouldn't be effective for FlaK 36's who have to manually adjust their fuzes. Anytime you put a human element into things, its going to be affected by human error.
I mean unless we want to ASSUME that:
1) enough people are going to care about defending a base to go help
2) 3 of those people are going to hop into '88s instead of aircraft or vehicles
3) those 3 are both well educated in high-level math and quick with the calculations so that they can coordinate and give each other usefull range information.
4) and that what are most likely 3 strangers are not only going to work together, but work together effectivly as a cohesive team.
then we better give the '88 gunners command of a battery of 3-4 guns, simmilar to the bomber-drones.
And I think that would be a good decision, because I don't know about you, but I think that betting on all 4 points happening simultaneously makes for some piss-poor odds.