Split accumulator 3-Way
Squeeze a tiny bit more juice from the original idea in #6336 which this is on top of.
https://tests.stockfishchess.org/tests/view/68dddd85fa806e2e8393c0b9
LLR: 2.95 (-2.94,2.94) <0.00,2.00>
Total: 156320 W: 40925 L: 40447 D: 74948
Ptnml(0-2): 427, 17330, 42172, 17800, 431
4-way doesn't look to be better than this.
https://tests.stockfishchess.org/tests/view/68dde19efa806e2e8393c0c1
closes https://github.com/official-stockfish/Stockfish/pull/6339
No functional change
Co-authored-by: M Stembera <m_stembera@yahoo.com>