ARM7 is a big category. Cortex-A9 and Cortex-M3 are among them, so you have a question.

Then cortex -A9 and cortex -M3 have different emphases. The former attaches importance to data processing and calculation and high performance, and is used in mobile phones and the like. The latter has low cost and good comprehensive performance, and is oriented to industrial and control applications.

So if you use Cortex-A9, learn Cortex-A9, which has nothing to do with M3. A9 part software, M3 part hardware.