When you eliminate the 3 partitions on the 600 gig old drive, you will then have to consider the sizes for any new partitions you create on it.
If it is split 200/200/200 now, you may decide on:
etc. Many people allow something like 60 to 100 for Windows (C) and then allocate the remainder to data--in one or more partitions. How you partition the remainder depends on your sense of organization and what type of data you generate.
Decide if you are going to keep your data on C. If you are, then a single C for the entire drive might be the best choice.