Form Advice

Good morning, I need some advice regarding forms after the passing of my in-laws. My mother-law passed away in 2020 however she did not have a will so her estate (including her house) was to be inherited by her husband and 3 children. My father-in-law applied for and received the grant of probate but did not change the house deeds into their names and unfortunately my father-in-law passed away last year. I am the executor of my father-in-law estate and the house is to be transferred to the 3 children so do I fill out forms AS1, AP1 & ID3? Also do I need to provide both grant of probate and/or death certificates for both? 

G A David - a complex scenario and one where you need to consider who now has the legal authority to deal with the property. That depends on the one fact you have not clarified and that is whether it is registered and if so is it in their joint names or solely the name of your late MIL. You say 'her house' so I am guessing she was the sole legal owner.
If that's the case then inheritance is one aspect and probate obtained by FIL would enable him to then transfer the legal ownership to the beneficiaries as appropriate. 
As he has now sadly passed away you will need probate for him and together the two probates can form what is known as a 'chain of representation'  - please do seek wider/legal advice re probate to confirm/clarify what is required and whether your late FUIL's executor can now deal with the property.
If they can then forms AP1, TR1 and ID1/ID3 for each party can be used to transfer the title/property to the new owner(s) as appropriate. The application should include certified copies of both probates

Thank you for the prompt and detailed reply Adam, much appreciated.
Regards, David