Can 2 cores call the same function concurrently?

Yes you can do this. Although there is only instance of the function, it can be executed simultaneously by 2 or 3 cores. Each core has its own RAM and stacks.

First thing to say is that cores 1 & 2 are about 18% faster than core0 due to the extra pipeline stage. This does not explain the timings you have seen though. Was each core using its own data RAM (DSPRx)? If a core uses variables in another core's local DSPRx RAM, the performance is hugely reduced.

If both try to access the same function simultaneously, there is a bottleneck accessing the FLASH memory. To prevent one core (i.e. the faster one) hogging the internal bus, "starvation prevention" is used to make sure that both cores get equal access rights to the flash. This is why both cores show the same run time. In situations like this, it would be a good idea to put frequently called functions into the PSPRx RAM for each core. This is not currently supported in the ShieldBuddy environment but it can be done from Eclipse.