In the US they are all rather imprecise and should only be used in an informal situation with people who don’t know much about the field. In that case I’d use ‘accountant-in-training’ or ‘accounting trainee’.

All sound fine. Since the first and third place two nouns side by side, you could separate them by hyphens if you wanted: "accountant-trainee" or "trainee-accountant". Another possibility is "accountant in training".

They're all more or less fine and none would sound strange.

You can say "Trainee Accountant." This is a common and clear way to describe your role as someone who is undergoing training in the field of accounting.