1. The full amount was $3.5 million, not $3 million. Thiel will be donating $500,000 in direct funds; the other $3 million is in the form of matching funds.
2. The $3.5 million will only count towards SENS research. It will not be included in the MPrize fund, which is a separate (though related) initiative. The MPrize will be awarded for creating strains of mice, or medical interventions for mice, that result in significant increases in mouse lifespan. SENS research is focussed on treating seven known contributing factors to aging. The two can go hand in hand (SENS research may produce an MPrize-winning mouse), but they are separate. Thiel's money can only be used for SENS research (conducted under the auspices of the Methuselah Foundation, the same organization that sponsors the MPrize, hence the probable basis for the confusion).
More information about SENS can be found here: