So to finally provide an update on this.
1. The 3/4 was not the faulty thing . It was the hose itself. The washer had started to disintegrate. Only realised it after the 3/4 thing was replaced and leak continued.
To be fair to myself, I was trying to work out cause of leak before and it felt like it was the coming from the 3/4 itself.
2. If you are replacing the 3/4 , replace the water hose along with it. Good practice!
All in all, was rather easy.
PS. Towels helped and emptying cistern in mandatory.