Generally... When upload is faster than download, that means there's a network fault? Download dropped to 5Mb/s .
It'll be your mast being over-contended. Go on webchat with three, tell them and they'll throw you a free month, I ended up getting 4 of them! (before I left).
I had 3 good months with three out of the 14 I was with them (Oct 2019-Dec 2019 was. absolutely amazing, even gave Virgin 100 a good run for its money) but as soon as Jan 2020 hit it just went down hill as more and more local customers signed up for the cheapest unlimited 4g Internet package.
(Maybe it's called "Three" because you should only expect 3 good months service?)
Three will no doubt be well aware of the issue at your mast, but if my experience is anything to go by you can complain daily and they won't do a thing to address it.