Can we account expenses by creating 3 level class (Main cost centre - cost centre --- sub cost centre?

Welcome aboard to the Community space, @umamaheswara-rao.

You can account for an expense with multiple classes by assigning it per line item. Having multiple classes in one transaction or one item isn't possible in QuickBooks Online (QBO). 

In regards to creating a sub-class, you can follow along with these steps. Here's how.

1. Go to the Gear icon, select All Lists.
2. Click Classes.
3. Select New. Give this class a name.
4. Thick Is sub-class box, and select the main class. You can nest up to five classes. 
5. Select Save.

You can also check out this link for more details about this: Create and manage classes in QuickBooks Online.

In case you're referring to creating a sub-account in your chart of accounts, please refer to this article for the complete process: Create subaccounts in your chart of accounts in QuickBooks Online.

I'd also recommend seeking advice from your accountant to ensure your accounts are well accounted for.

Furthermore, I'm adding this resource that you can utilize in matching your accounts accurately: Reconcile an account in QuickBooks Online.

I'm all ears if you need more help with your expenses or with your account. Keep me in the loop by leaving a reply. I'll be here ready to back you up!