Sun-grazing comet C/1999 X3 was discovered by SOHO and does not belong to any known Sun-grazer group. The comet resides in near-Earth space and no cometary outgassing component constants are available. Using recent orbital data on May 2012, we numerically followed a nominal orbit of the comet and put all planets as perturbing bodies. We find that C/1999 X3 will plunge into the sun in only about 2100 yrs. This lifetime is roughly a quarter that of the known sun-grazer comet 96P/Machholz 1. Involving an adopted 96P model of cometary outgassing and relativity correction into the equations of motion of the comet will insignificantly shorten its lifetime, before impact. We generated about 3500 orbital clones of C/1999 X3 to analyze orbital variations of the comet. Results show that 95% clones will impact the Sun over the lifetime. This high solar impact probability offers a study of cometary dynamics in near-Earth space for which it may indicate a new group of Sun-grazer.