Reading the ports is non-destructive, so all you need to do is mask off the lower 5 bits that interest you with AND OR operations.

When you write to the other three bits, you can mask THEM off from affecting the other port bits with AND and OR operations.

You can read-write-modify with ANL and ORL, so that you read the actual pin value, not the output latch. I am not sure how you will achieve that in C.

Steve