Hello there, @eric9.
I'm here to help guide you with the process so you'll be able to file VAT return for the three quarters successfully.
If it's you first time to file the VAT return, then, you can just modify the starting and ending date of the filing period using the correct date (01/07/18-31/03/19). Please ensure that the transactions are added in QuckBooks on the said period.
On other hand, if you've filed VAT prior to the said date, I recommend contacting our Support Team for additional assistance. For the support's contact information, you may check it here: Contact QuickBooks.
To learn more about submitting a VAT return in QuickBooks, you can check this article: Submit a VAT return in QuickBooks.
Please know that I'm just a post away to help you if you have any other questions about filing VAT return, just add a comment below. Wishing you the best!