In threads i feel the memory of the same process would be divided equally amongst the threads.

Well, sort of, but the process address space which they all share is virtual memory. If you mean 'real' memory then the thread will use as much or as little as the OS allows it to have at the time - and that will likely depend as much as what else is running as anything else.

s/memory/CPU use/g
Yes, but only if you design it that way by distributing the task or the data evenly between threads.