How do you know that they are using any of your IP (code)? I presume that they approached you, originally, to create software for them, but the idea was theirs

How do you know that they are using any of your IP (code)? I presume that they approached you, originally, to create software for them, but the idea was theirs. If they terminate a contract with you, however unsatisfactorily, they are still free to start over with another dev agency. 3 years is a long time in terms of what can be delivered in software. The system doesn't sound all that difficult to replicate.