I thought that I would respond because nobody else has.
I can’t comment on Threeholes specifically but I live near Downham Market and drive through Upwell/Outwell occasionally. As far as I am aware Threeholes isn’t known for flooding and I can’t recall it ever being reported locally. Fenland areas have centuries of experience of managing water levels and so are less prone to flooding than you would expect, although like lots of areas, climate change could make it more difficult in the future.
Welney, which is fairly close, floods at least every year but on one road and is expected to the point that there are warning lights ahead of the road, but every year at least one driver ignores them and becomes stranded, with photos published in the news. Houses generally aren’t affected and it is just an annoying fact of life for those who live there and have to take a longer detour.
Generally, Norfolk roads are good in the winter, if you don’t mind a bit of mud, and the flatness makes driving easier when it’s icy.
If you have children who will require secondary school, that is currently a problem, with a lot of Upwell/Outwell children not getting places at the catchment Downham Academy this year and expected to be bused miles away. The school has historically been undersubscribed with a poor reputation but recent improvements, together with a boom in local house building, has changed that. Marshland High School, just across the border into Cambridgeshire, has been/is one of the most coveted local schools and so you may wish to try to move into the catchment area for that instead.