A post-World War 1 British battlecruiser project, 4 ships of this class were ordered but none were apparently laid down before the project was cancelled due to Washington Naval Treaty, materials and ideas meant for this class (alongside with those meant for their battleship counterparts: N3-class) were ultimately used in design and construction of Nelson-class battleships. Even though G3s were officially classified as battlecruisers in design they were closer to what we would today consider fast battleships; featuring main battery of 9 406mm guns mounted in three triple turrets, 16 155mm secondary guns in 8 twin turrets, 356mm (magazines) to 305mm (machinery spaces) thick armored belt, 203mm to 73mm thick armored deck (areas around magazines being once again significantly more heavily armored than those around machinery spaces), heavily armored turrets  (444mm thick faces, 330mm to 229mm sides, 203mm roof), and armored bulkheads 305mm to 254mm thick, with design speed of 32 knots and full displacement of 54k tons. G3's most notable feature (and the one which allowed to it to as heavily armored as it was) was its rather unorthodox turret placement; 2 turrets (A and B) forward with B turret superfiring over A turret, one turret midship (X), this layout allowed the armor to be concentrated in much smaller area than it would had been possible if a more conventional turret layout had been used. For more info about G3s and their design history both "Wikipedia":https://en.wikipedia.org/wiki/G3_battlecruiser and "Warship Projects":https://warshipprojects.com/2017/09/27/washington-cherry-trees-ii-part-3/ have decent articles about them.
g3 battlecruiser