Back in 2014 Channel 5 obtained the UK rights to “Gotham” with Netflix getting the show once the linear broadcast was complete. This worked well up until season 3 when Channel 5, for whatever reason, never actually aired season 3. Eventually Netflix were allowed to show season 3 but no one still has any idea what happened to it on Channel 5.
Gotham now has a new home in the UK and E4 will be airing it from April. The deal with Netflix is unchanged and season 4 will be added to Netflix UK at some point after it has aired on E4.
Season 3 has still not been shown on terrestrial TV in the UK.