There is a similar post to this one here.
In a nutshell you’d need to set the dates for the first 2 months in the period and commit the return (without submitting to HMRC). Then change the FR VAT % in your VAT settings and commit another return for the remaining month. You would then need to total boxes 1-9 from each of these returns and submit manually via the HMRC online portal.
The other thing you can do is run the report as above for the two periods, note down the values (boxes 1-9) without commiting then run again for the 3 months and use the adjustment boxes to get to the correct net figures. You can then adjust and submit electronically to HMRC.
Hope that helps!