From what I understand from your question, you want to use multiple subdomains, and point these to different droplets:
You do this in your DNS settings. Create multiple droplets, each with their own IP address. Then in your DNS records, create an A record with the value of your subdomain, pointing to the IP address of the relevant droplet.

So for example, I have droplet with IP 1.1.1.1, this will have DNS record A, which has value subdomain1 and in the NAME field, and in the VALUE field, goes the IP address of the droplet. So it will be subdomain1.mydomain.com, then repeat for subdomain2, etc.
Hope this helps…