Hi Anna
DS is 8 months as well, like you I have been offering him water since he was 4 months and able to hold a sippy.
Initially he would spit it back out again, but since I have started mixing it with
a little baby dilute juice he will take it. Recently he even took plain water from his sippy.
I have give him water or juice in his sippy only he holds the cup himself and takes what he wants during and after meals i.e lunch and dinner.
The key is perserverance.

If you are looking to cut to 3 bottles, usually your LO will instigate this by fussing with the 10.30 bottle - when that happens your bottle feeding times could be; 0645/13.30 /18.30
HTH's