The reactions of grignard reagent
The third Grignard reagent molecule could extract a proton from the methylene group which is in an electron withdrawing environment.  This type of enhanced acididity on carbon is referred to by the term "carbon acid" .  

While this reference deals with carbonyl groups, conjugation with a phenyl ring can do the same thing to the methylene group.

We do not see any change of the methylene group in the final product because it's just deprotonated, and the acid workup brings the proton back.  We would, however, see the third Grignard reagent being protonated to its parent hydrocarbon, in this case methane which bubbles off.