I would put the tent and mat sidways, tent frontmost.
Put the sleeping bag inside a drybag.
Lash down with some strong elastic chord or some medium width road inner tube lengths.
Tents are often packed wet so you dont want another bag.
Grounsheet can be rolled around tent or mat or just put ontop.
You have to experiment to se what works.
I use the handle of a drybag to thread lashing through so my bag wont slip sideways