You are seeing both messages on screen because you are actually in both situations at the same time.
The maximum amount of Social Security withholdings is $8853.60 per taxpayer. If your box 4 withholdings on the combined W-2 that covers employers A and B is greater than $8853.60, then that portion of the excess is being treated as coming from a single employer who withheld too much, which is not allowed.
The amount that is being reported on your Schedule 3 Line 11 should be the excess amount that would have been withheld if the first W-2 had stopped withholding at the maximum allowed limit. Having Schedule 3 Line 11 be only the amount of excess due to employer C is correct since it appears that the other W-2 is wrong and having too much withheld from a single employer cannot be automatically refunded.
Even though your situation is not truly a single employer, having A and B combined on a single W-2 treats it that way.
You will need to follow the advice in the following TurboTax article under the Single-Employer section to get the excess refunded:
Can I get a refund for excess Social Security tax withheld?
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