Great decision on moving to DigitalOcean. I moved my personal workloads to here some years ago and have loved the platform while using it in various capacities.

What you are trying to achieve is quite possible and not without the possibility of achieving it through various methods.

One approach I would take is as follows for a single droplet setup using the DigitalOcean DNS + Firewalls etc…: - Note: No admin panel outside DigitalOcean is used

• Create a droplet that suited the task. Could start with the $5 1GB/1CPU/25GB - The droplet size can later be increased as required.
  Note: The droplet does need to be powered off when scaling and will result in an outage for the website(s)
  https://www.digitalocean.com/community/tutorials/how-to-create-your-first-digitalocean-droplet

• Assign the droplet a Floating IP
  https://www.digitalocean.com/community/tutorials/how-to-use-floating-ips-on-digitalocean

• Add the domains for the 3 sites to DigitalOcean DNS and add the appropriate entries for each domain. I at this point as a subdomain of dev.* to each domain - i.e. dev.c3po.com that points to the Floating IP
  https://www.digitalocean.com/community/tutorials/how-to-set-up-a-host-name-with-digitalocean

• For revers DNS/PTR Records if required
  https://serverpilot.io/community/articles/how-to-set-ptr-records-and-reverse-dns.html

• Setup Firewall policy and add the droplet
  https://www.digitalocean.com/community/tutorials/an-introduction-to-digitalocean-cloud-firewalls

• Create a linux user account for each site and setup the home directory as desired.

  EXAMPLE: /home/c3po
  - home
  - - c3po
  - - - public_html
  - - - logs
  - - - backups
  

• Setup the LEMP stack
  https://www.digitalocean.com/community/tutorials/how-to-install-linux-nginx-mysql-php-lemp-stack-in-ubuntu-16-04

• Configure PHP Pool for each account
  https://www.digitalocean.com/community/tutorials/how-to-host-multiple-websites-securely-with-nginx-and-php-fpm-on-ubuntu-14-04
  Note: this is for Ubuntu:14.04 however the process is the same for Ubuntu:16.04

  EXAMPLE: /etc/php/7.0/fpm/pool.d/c3po.conf
  [c3po]
  user = c3po
  group = c3po
  listen = /var/run/php/php7.0-fpm-c3po.sock
  listen.owner = nginx
  listen.group = nginx
  request_terminate_timeout = 300
  pm = dynamic
  pm.max_children = 5
  pm.start_servers = 2
  pm.min_spare_servers = 1
  pm.max_spare_servers = 3
  chdir = /
  

• Clone the first first site into the respective user account and web root

• Add NGINX entry

  EXAMPLE: /etc/nginx/conf.d/c3po.com.conf
  upstream php7 {
  server unix:/var/run/php/php7.0-fpm-c3po.sock;
  }
  server {
  listen       80;
  server_name  dev.c3po.com c3po.com www.c3po.com;
  charset utf-8;
  access_log  /home/c3po/logs/access.log;
  error_log     /home/c3po/logs/error_log;
  location / {
  root   /home/c3po/public_html;
  index  index.php index.html index.htm;
  }
  location ~ \.php$ {
  fastcgi_pass php7;
  fastcgi_split_path_info ^(.+\.php)(/.+)$;
  fastcgi_index index.php;
  include fastcgi_params;
  fastcgi_param SCRIPT_FILENAME $document_root/$fastcgi_script_name;
  fastcgi_read_timeout 300;
  }
  }
  

• Add an entry to the DNS zone file for dev.* - dev.c3po.com to your registrar or previous hosting zone file that points to the Droplets Floating IP - Test everything out to ensure it is working as expected. Remove dev.* entry from the DigitalOcean DNS and NGINX Config once complete.

• Update your domain(s) to DigitalOceans nameservers ns1.digitalocean.com, ns2.digitalocean.com, ns3.digitalocean.com

• Secure the LEMP stack with LetsEncrypt once domain propagation is complete
  https://www.digitalocean.com/community/tutorials/how-to-secure-nginx-with-let-s-encrypt-on-ubuntu-16-04

Rinse and repeat for each domain as required. As stated before this is one possible way. Depending on the application you are running some details may need to be adjusted accordingly.

Another approach - https://blog.ssdnodes.com/blog/tutorial-using-docker-and-nginx-to-host-multiple-websites/

Setting up Vanity Name Servers - https://www.digitalocean.com/community/tutorials/how-to-create-vanity-or-branded-nameservers-with-digitalocean-cloud-servers

To avoid downtime you could design an infrastructure that spans multiple droplets with redundancy systems built in - https://www.digitalocean.com/community/tags/high-availability?type=tutorials