All Alarm.com systems would have their own account. If you have 3 systems they would be 3 separate Alarm.com accounts. They are likely connected via Multi-System-Access in Alarm.com. I couldn’t speak to any consolidation your prior provider would have with billing accounts.
Are all of the systems GC2s? The steps to get started with an existing GC2 are found here.
Basically in our website you would just purchase a subscription for each system, then with an unregistered module you could activate each of them and set up multi-system-access again in Alarm.com. This standard process would result in brand new Alarm.com accounts, so rules and notifications would need to be recreated.
Optionally a transfer process exists which would just transfer the existing Alarm.com accounts to Surety, but the limitation there is that the current service provider must support this and initiate the transfer. The exact steps for that option are discussed here.