For 3 He submonolayer solid adsorbed on Grafoil, the evolutions of the adsorption structure and of the competition of multiple-spin-exchange interactions with increasing areal density are discussed. The discrepancy of the heat-capacity exponent from the normal value, 22, is considered to show the strength of the competition. We give various discussions to explain the density dependence of the competition, the sudden change of the exchange frequency, and other observations. There we propose the

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... tructural phase diagram constructed from the simulations and the possible zero-point vacancies.