Writing a series of papers
It sounds to me like you are trying to game the system by publishing Part A three times simultaneously with revisions to the wording to claim it is not self-plagiarized.

Assuming Part A is important, which it seems to be because you are describing it as a new technology, you seem to be trying to get three journals to think they are each the first to publish that new technology, when in fact they are not. This is unethical.

If all the applications are relevant, you should publish whichever your favorite one is with Part A, and then submit separate papers citing that paper that show the results with the other applications. These works will likely repeat some of Part A, but only as much as is needed to explain it for that application.

However, this is something best discussed with your academic advisor.